#include <bits/stdc++.h>
using namespace std;
int main() {
  int fac, a, inshi, n, m, h, i, sum = 0;
  double d = 1;
  scanf("%d", &n);
  scanf("%d", &m);
  scanf("%d", &h);
  h--;
  for (i = 0; i < m; i++) {
    scanf("%d", &a);
    sum += a;
    if (i == h) fac = a;
  }
  fac--;
  n--;
  sum--;
  inshi = sum - fac;
  if (sum < n) {
    cout << -1.0;
    return 0;
  }
  if (fac == 0) {
    cout << 0.0;
    return 0;
  }
  if (inshi < n) {
    cout << 1.0;
    return 0;
  }
  for (i = 0; i < n; i++) {
    d *= (inshi - i);
    d /= (double)(sum - i);
  }
  cout << 1 - d;
  return 0;
}
