#include <bits/stdc++.h>
using namespace std;
int n, m, h, sh, s, i, t, v, niz;
double res, r;
int main() {
  cin >> n >> m >> h;
  for (i = 1; i <= m; i++) {
    cin >> t;
    s += t;
    if (i == h) {
      s--;
      sh = t - 1;
    }
  }
  n--;
  if (s == n && sh != 0) {
    cout << 1 << endl;
    return 0;
  }
  if (s < n) {
    cout << -1 << endl;
    return 0;
  }
  res = 1;
  for (i = 0; i < n; i++) res = (res * (s - sh - i)) / (s - i);
  printf("%.8f", 1 - res);
  return 0;
}
