#include <bits/stdc++.h>
using namespace std;
long double fac[100100];
long double ans;
long double inf = 1e99;
int n, m, k;
int main() {
  scanf("%d%d%d", &n, &m, &k);
  for (int i = 1; i <= m; i++) fac[i] = fac[i - 1] + log(i);
  for (int r = 0; r <= n; r++) {
    for (int c = 0; c <= n; c++) {
      int tot = (r + c) * n - r * c;
      if (k < tot) continue;
      long double cur = fac[n] - fac[r] - fac[n - r] + fac[n] - fac[n - c] -
                        fac[c] + fac[k] - fac[k - tot] - fac[m] + fac[m - tot];
      ans = min(inf, ans + exp(cur));
    }
  }
  cout << fixed << setprecision(15) << ans << endl;
  return 0;
}
