C. Тир

#include <bits/stdc++.h>

using namespace std;

const int N = 19;

double p[N][N];

int main () {
    int n, m, k; cin >> n >> m >> k;
    double t = n * m;
    p[0][0] = 1;
    for (int j = 1; j <= k; j++) {
        for (int i = 0; i <= min (j, (int) t); i++) {
            p[i][j] = (i > 0 ? p[i - 1][j - 1] * ((t - (i - 1)) / t) : 0) + p[i][j - 1] * (i / t);
        }
    }
    double ans = 0;
    for (int i = 1; i <= k; i++) {
        ans += p[i][k] * i;
    }
    cout << fixed << setprecision (12) << ans << endl;
    return 0;
}