#include <bits/stdc++.h>
using namespace std;
double f(int s, int x, int n) {
  double t = 1;
  for (int i = 1; i < n; i++) {
    t = (t * (s + 1 - x - i)) / (s - i);
  }
  return t;
}
int main() {
  cout << setprecision(6);
  int n, m, h, s = 0;
  cin >> n >> m >> h;
  int a[m];
  for (int i = 0; i < m; i++) {
    cin >> a[i];
    s = s + a[i];
  }
  if (n > s) {
    cout << -1;
  } else {
    cout << 1 - f(s, a[h - 1], n);
  }
  return 0;
}
