#include<stdio.h>
using namespace std;

double N,D,M;

int main()
{
    scanf("%lf%lf%lf",&N,&M,&D);

    double ans = 0;
    if(D == 0)ans = N * (M - 1) / N / N;
    else ans = 2 * (N - D) * (M - 1) / N / N;
    printf("%.10lf\n",ans);
}
