#include <iostream>
#include <cstdio>

using namespace std;

int main(){
	int n, m, d;
	while(cin >> n >> m >> d){
		long long sum = (d == 0 ? n : 2 * max(0, n - d));
		double div = n;
		printf("%.10lf\n", sum*(m-1)/div/div);
	}
}