#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  while (cin >> n >> m >> h) {
    int S = 0, k[m];
    for (int i = 0; i < m; i++) {
      cin >> k[i];
      S += k[i];
    }
    if (n > S)
      cout << "-1.0" << endl;
    else {
      n--;
      h--;
      S--;
      k[h]--;
      double x = 1, p = 1;
      for (int i = 0; i < n; i++) p *= 1.0 * (S - i - k[h]) / (S - i);
      printf("%.10lf\n", x - p);
    }
  }
  return 0;
}
