#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  --h;
  int s = 0, q = 0;
  for (int i = 0; i < m; ++i) {
    int x;
    cin >> x;
    s += x;
    if (i == h) {
      q = x;
    }
  }
  long double res;
  if (s < n) {
    res = -1.0;
  } else {
    if (n - 1 > s - q) {
      res = 1.0;
    } else {
      res = 1.0;
      for (int i = 0; s - 1 - i > s - q; ++i) {
        res *= 1.0 * (s - n - i) / (s - 1 - i);
      }
      res = 1.0 - res;
    }
  }
  printf("%.9lf\n", res);
  return 0;
}
