#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0;i < n;i++)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end())
#define VRSORT(v) sort(v.rbegin(), v.rend())
#define ll long long
#define pb(a) push_back(a)
#define MOD 1000000007
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;
typedef vector<unsigned int>vec;
typedef vector<vec> mat;
//typedef tuple<ll, ll, ll> T;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
void add(ll& a, ll b){a = (a+b) % MOD;}
int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};
double r, n, m;
double cut(int i)
{
if (i <= 0 or i >= n) {
return (0);
}
else {
return (2*(sqrt(pow(r, 2)-pow(abs(i*(2*r/n)-r),2))));
}
}
int main(){
cin.tie(0);
ios::sync_with_stdio(false);
cin >> r >> n >> m;
long double ans = 0;
for (int i = 1; i <= n + m - 1; i++) {
ans += max(cut(i), cut(i-m));
}
printf("%.10lf\n", (double)ans);
}