#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  int *arr = new int[m + 1];
  for (int i = 1; i <= m; i++) cin >> arr[i];
  int sum = 0;
  for (int i = 1; i <= m; i++) sum += arr[i];
  if (sum < n) {
    cout << -1.0;
    return 0;
  }
  if (sum == m) {
    if (arr[h] == 1)
      cout << 0;
    else
      cout << 1;
    return 0;
  }
  int sum2 = sum - arr[h];
  sum -= 1;
  arr[h] -= 1;
  n -= 1;
  double res = 1;
  double up = sum2;
  double down = sum;
  if (n == 0) {
    cout << 0;
    return 0;
  }
  if (sum2 < n) {
    cout << 1;
    return 0;
  } else {
    for (int i = 1; i <= n; i++) {
      double x = up / down;
      res *= x;
      up--;
      down--;
    }
  }
  printf("%.8f", 1 - res);
}
