#include <bits/stdc++.h>
using namespace std;
int main() {
  cout.precision(10);
  int n, m, h, s, te, a;
  cin >> n >> m >> h;
  s = 0;
  for (int i = 1; i <= m; i++) {
    cin >> te;
    s += te;
    if (i == h) a = te;
  };
  a--;
  s--;
  n--;
  if (s < n) {
    cout << -1;
    return 0;
  };
  double sol = 1;
  for (int i = 1; i <= a; i++)
    sol = sol * (((s - n - a + i) * 1.0) / ((s - a + i) * 1.0));
  sol = 1.0 - sol;
  cout << sol;
  return 0;
};
