#include <bits/stdc++.h>
using namespace std;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define REP(i, n) FOR(i,0,n)

int main()
{
    double R, H, D;
    cin >> R >> H >> D;

    constexpr int Z = 1 << 10;
    double dz = 1.0 / Z;
    double ret = 0.0;
    REP(t, Z) {
        double u = 1.0 - (dz * dz * (t + 0.5) * (t + 0.5));
        double dd = 2 * dz * dz * (t + 0.5);
        if (D / (2 * R) > u) continue;
        double theta = acos(D / (2 * R) / u);
        ret += u * u * (theta * 0.5 - sin(theta * 2) * 0.25) * dd;
    }
    cout << fixed << setprecision(20) << ret * 4 * R * R * H << '\n';
}