#include <iomanip>
#include <iostream>
using namespace std;
int n, m, d;
int main() {
	cin >> n >> m >> d;
	cout << fixed << setprecision(15) << 1.0 * (d == 0 ? n : (n - d) * 2) * (m - 1) / n / n << '\n';
	return 0;
}