#include <bits/stdc++.h>
using namespace std;
int m, n, h, departments[1000], sum;
double p = 1;
int main() {
  cin >> n >> m >> h;
  for (int i = 0; i < m; i++) {
    cin >> departments[i];
    sum += departments[i];
  }
  for (int i = 0; i < n - 1; i++)
    p = p * (sum - departments[h - 1] - i) / (sum - i - 1);
  if (n > sum) p = 2;
  cout << 1 - p;
  return 0;
}
