#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  int team[m];
  int sum = 0;
  for (int i = 0; i < m; ++i) {
    cin >> team[i];
    sum += team[i];
  }
  if (sum < n)
    cout << -1 << "\n";
  else {
    team[h - 1]--;
    n--;
    sum--;
    double ans = 1;
    for (int i = 0; i < team[h - 1]; ++i) {
      ans *= (sum - n - i) * 1.0 / (sum - i);
    }
    cout << 1 - ans << "\n";
  }
  return 0;
}
