文件目录

#include <bits/stdc++.h>
using namespace std;
double l1, l2, n, c;
bool pd(double x)
{
    double r = l1 / (2 * sin(x / 2));
    return r * x >= l2;
}
int main()
{
    cin >> l1 >> n >> c;
    l2 = (1 + n * c) * l1;
    double l = 0, r = 3.14159265359, x;
    while (fabs(l - r) >= 1e-10)
    {
        x = (l + r) / 2;
        if (pd(x))
            r = x;
        else
            l = x;
    }
    printf("%.3lf", l2 / x * (1 - cos(x / 2)));
    return 0;
}