import java.io.IOException;
import java.io.InputStream;
import java.math.BigDecimal;
import java.util.*;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.function.Supplier;
public class Main {
void run() {
int r = ni();
int n = ni();
int m = ni();
double sum = 0;
BigDecimal SUM = BigDecimal.ZERO;
double nagasa = (double) 2 * r / n;
for (int i = 1; i <= n + m; ++i) {
double max = 0;
BigDecimal MAX = BigDecimal.ZERO;
if (0 < i && i < n) {
double I = Math.abs((double) n / 2 - i);
double hen = I * nagasa;
BigDecimal R = new BigDecimal(r);
BigDecimal H = new BigDecimal(hen);
BigDecimal S = R.pow(2).subtract(H.pow(2));
if (MAX.compareTo(S) < 0) {
MAX = S;
}
max = Math.max(max, 2 * Math.sqrt(r * r - hen * hen));
}
int j = i - m;
if (0 < j && j < n) {
double J = Math.abs((double) n / 2 - j);
double hen = J * nagasa;
BigDecimal R = new BigDecimal(r);
BigDecimal H = new BigDecimal(hen);
BigDecimal S = R.pow(2).subtract(H.pow(2));
if (MAX.compareTo(S) < 0) {
MAX = S;
}
max = Math.max(max, 2 * Math.sqrt(r * r - hen * hen));
}
// debug(i, j, max);
max = 2 * Math.sqrt(MAX.doubleValue());
sum += max;
}
System.out.printf("%.9f",sum);
}
Scanner sc = new Scanner(System.in);
public static void main(String[] args) {
new Main().run();
}
int ni() {
return Integer.parseInt(sc.next());
}
void debug(Object... os) {
System.err.println(Arrays.deepToString(os));
}
class BIT<T> {
int n;
ArrayList<T> bit;
BiFunction<T, T, T> bif;
/**
* 1-indexed なBinary Indexed Treeを構築する
*
* @param n 容量
* @param bif 適用させる関数
* @param sup 初期値
*/
BIT(int n, BiFunction<T, T, T> bif, Supplier<T> sup) {
this.n = n;
bit = new ArrayList<>(n + 1);
for (int i = 0; i < n + 1; ++i) {
bit.add(sup.get());
}
this.bif = bif;
}
/**
* iの位置の値をvで更新する
*
* @param i index
* @param v 新しい値
*/
void update(int i, T v) {
for (int x = i; x <= n; x += x & -x) {
bit.set(x, bif.apply(bit.get(x), v));
}
}
/**
* クエリー
*
* @param defaultValue 初期値
* @param i index
* @return [1, i]までfを適用した結果
*/
T reduce(T defaultValue, int i) {
T ret = defaultValue;
for (int x = i; x > 0; x -= x & -x) {
ret = bif.apply(ret, bit.get(x));
}
return ret;
}
}
long MOD = 1_000_000_007;
/**
* 繰り返し2乗法を用いたべき乗の実装
*
* @return a^r (mod 1,000,000,007)
*/
long pow(long a, long r) {
long sum = 1;
while (r > 0) {
if ((r & 1) == 1) {
sum *= a;
sum %= MOD;
}
a *= a;
a %= MOD;
r >>= 1;
}
return sum;
}
/**
* 組み合わせ
* O(n)
*
* @return {}_nC_r
*/
long C(int n, int r) {
long sum = 1;
for (int i = n; 0 < i; --i) {
sum *= i;
sum %= MOD;
}
long s = 1;
for (int i = r; 0 < i; --i) {
s *= i;
s %= MOD;
}
sum *= pow(s, MOD - 2);
sum %= MOD;
long t = 1;
for (int i = n - r; 0 < i; --i) {
t *= i;
t %= MOD;
}
sum *= pow(t, MOD - 2);
sum %= MOD;
return sum;
}
double GOLDEN_RATIO = (1.0 + Math.sqrt(5)) / 2.0;
/**
* 黄金分割探索
*
* @param left 下限
* @param right 上限
* @param f 探索する関数
* @param comp 上に凸な関数を探索するときは、Comparator.comparingDouble(Double::doubleValue)
* 下に凸な関数を探索するときは、Comparator.comparingDouble(Double::doubleValue).reversed()
* @return 極値の座標x
*/
double goldenSectionSearch(double left, double right, Function<Double, Double> f, Comparator<Double> comp) {
double c1 = divideInternally(left, right, 1, GOLDEN_RATIO);
double c2 = divideInternally(left, right, GOLDEN_RATIO, 1);
double d1 = f.apply(c1);
double d2 = f.apply(c2);
while (right - left > 1e-9) {
if (comp.compare(d1, d2) > 0) {
right = c2;
c2 = c1;
d2 = d1;
c1 = divideInternally(left, right, 1, GOLDEN_RATIO);
d1 = f.apply(c1);
} else {
left = c1;
c1 = c2;
d1 = d2;
c2 = divideInternally(left, right, GOLDEN_RATIO, 1);
d2 = f.apply(c2);
}
}
return right;
}
/**
* [a,b]をm:nに内分する点を返す
*/
double divideInternally(double a, double b, double m, double n) {
return (n * a + m * b) / (m + n);
}
long gcd(long a, long b) {
if (b == 0) {
return a;
}
return gcd(b, a % b);
}
int gcd(int a, int b) {
if (b == 0) {
return a;
}
return gcd(b, a % b);
}
long lcd(long a, long b) {
return (a / gcd(a, b)) * b;
}
int lcd(int a, int b) {
return (a / gcd(a, b)) * b;
}
/**
* http://qiita.com/p_shiki37/items/65c18f88f4d24b2c528b
*/
static class FastScanner {
private final InputStream in;
private final byte[] buffer = new byte[1024];
private int ptr = 0;
private int buflen = 0;
public FastScanner(InputStream in) {
this.in = in;
}
private boolean hasNextByte() {
if (ptr < buflen) {
return true;
} else {
ptr = 0;
try {
buflen = in.read(buffer);
} catch (IOException e) {
e.printStackTrace();
}
if (buflen <= 0) {
return false;
}
}
return true;
}
private int readByte() {
if (hasNextByte()) return buffer[ptr++];
else return -1;
}
private static boolean isPrintableChar(int c) {
return 33 <= c && c <= 126;
}
private void skipUnprintable() {
while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
}
public boolean hasNext() {
skipUnprintable();
return hasNextByte();
}
public String next() {
if (!hasNext()) throw new NoSuchElementException();
StringBuilder sb = new StringBuilder();
int b = readByte();
while (isPrintableChar(b)) {
sb.appendCodePoint(b);
b = readByte();
}
return sb.toString();
}
public long nextLong() {
if (!hasNext()) throw new NoSuchElementException();
long n = 0;
boolean minus = false;
int b = readByte();
if (b == '-') {
minus = true;
b = readByte();
}
if (b < '0' || '9' < b) {
throw new NumberFormatException();
}
while (true) {
if ('0' <= b && b <= '9') {
n *= 10;
n += b - '0';
} else if (b == -1 || !isPrintableChar(b)) {
return minus ? -n : n;
} else {
throw new NumberFormatException();
}
b = readByte();
}
}
}
}