#include <bits/stdc++.h>
int n, m, k, r, i, j;
long double f[1 << 18], a;
int main() {
  scanf("%d%d%d", &n, &m, &k);
  for (i = 1; i <= m; ++i) f[i] = f[i - 1] + log(i);
  for (i = 0; i <= n; ++i)
    for (j = 0; j <= n; ++j) {
      r = n * (i + j) - i * j;
      a += k >= r ? exp(2 * f[n] - f[i] - f[n - i] - f[j] - f[n - j] +
                        f[m - r] - f[k - r] - f[m] + f[k])
                  : 0;
    }
  a = a > 1e99 ? 1e99 : a;
  printf("%.9f\n", (double)a);
}
