#include <bits/stdc++.h>
using namespace std;
int s[2000];
int main() {
  cout << setprecision(20) << fixed;
  int n, m, h;
  cin >> n >> m >> h;
  int sum = 0;
  for (int i = 1; i <= m; ++i) cin >> s[i], sum += s[i];
  int S = sum - s[h];
  int R = sum - 1;
  if (S < n - 1 && R >= n - 1 && s[h] > 1)
    cout << 1 << endl;
  else if (n - 1 > S)
    cout << -1 << endl;
  else if (s[h] == 1)
    cout << 0 << endl;
  else {
    long double p = 1;
    for (int i = 1; i <= n - 1; ++i) {
      p *= S / (long double)R;
      S--, R--;
    }
    cout << 1 - p << '\n';
  }
  return 0;
}
