#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, m, h, mh;
  cin >> n >> m >> mh;
  long long int sm = 0;
  long long int d;
  for (long long int i = 1; i <= (long long int)m; i++) {
    cin >> d;
    sm += d;
    if (i == mh) h = d;
  }
  if (n > sm) {
    printf("-1");
    return 0;
  }
  double ans = 1;
  for (long long int i = sm - 1; i > sm - n; i--) ans = ans / i;
  for (long long int i = sm - h; i > sm - h - n + 1; i--) ans = ans * i;
  printf("%.10f", 1 - ans);
  return 0;
}
