#include "math.h"
#include "stdio.h"

int main()
{
    double R, H, D;
    scanf("%lf%lf%lf", &R, &H, &D);

    const int Z = 1 << 9;
    const double dz = 1.0 / Z;

    double ret = 0.0;
    const double T = D / (2 * R);
    const double lo = T;

    for (int t = 0; t < Z; t++) {
        double u = 1.0 - (dz * dz * (t + 0.5) * (t + 0.5)) * (1 - lo);
        double dd = 2 * dz * dz * (t + 0.5) * (1 - lo);
        double theta = acos(T / u);
        ret += u * u * (theta * 0.5 - sin(theta * 2) * 0.25) * dd;
    }
    printf("%.10lf\n", ret * 4 * R * R * H);
}