#include <bits/stdc++.h>
using namespace std;
long double n, m, h;
int main() {
  cin >> n >> m >> h;
  int sum = 0;
  vector<int> s(m + 1);
  h--;
  for (int i = 0; i < m; i++) {
    cin >> s[i];
    sum += s[i];
  }
  s[h]--;
  sum--;
  n--;
  if (sum < n) {
    cout << -1;
    return 0;
  }
  if (sum - s[h] < n) {
    cout << 1;
    return 0;
  }
  long double x = 1;
  for (long double i = 0; i < n; i++) {
    x *= (sum - s[h] - i) / (sum - i);
  }
  cout << 1 - x;
}
