#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1.0);
const double eps = 1e-11;
const int INF = 101e7;
template <class T>
T sqr(T first) {
  return first * first;
}
template <class T>
T abs(T first) {
  return (first < 0) ? -first : first;
}
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  n--;
  int sh = 0, sum = 0;
  for (int _b = (m), i = (1); i <= _b; i++) {
    if (i == h) {
      cin >> sh;
      sh--;
    } else {
      int k;
      cin >> k;
      sum += k;
    }
  }
  if (n > sh + sum) {
    cout << "-1\n";
    return 0;
  }
  double c = 1;
  for (int i = 0, _n = (n); i < _n; i++) {
    c *= (double(sum) / (sum + sh));
    sum--;
  }
  printf("%.8f\n", 1 - c);
  return 0;
}
