#include <bits/stdc++.h>
using namespace std;
int team;
int fac;
int others;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  team = n - 1;
  double res;
  for (int i = 1; i <= m; i++) {
    if (i == h) {
      cin >> fac;
      fac--;
    } else {
      int a;
      cin >> a;
      others += a;
    }
  }
  if (others + fac < team) {
    cout << "-1.0" << endl;
    return 0;
  }
  if (others < team) {
    cout << "1.0" << endl;
    return 0;
  }
  res = 1.0;
  while (team) {
    res = (res * others) / (others + fac);
    team--;
    others--;
  }
  res = 1.0 - res;
  printf("%.8lf\n", res);
  return 0;
}
