#include <bits/stdc++.h>
using namespace std;
long long a[1111];
int main() {
  ios::sync_with_stdio(0);
  ;
  long long n, m, h;
  cin >> n >> m >> h;
  long long sum = 0;
  for (int i = 0; i < m; i++) {
    cin >> a[i];
    sum += a[i];
  }
  if (sum < n) {
    cout << -1 << endl;
    return 0;
  }
  double r = 1;
  for (int i = 0; i < n - 1; i++) {
    r = r * double(sum - a[h - 1] - i) / double(sum - 1 - i);
  }
  double res = 1 - r;
  printf("%.7lf", res);
  return 0;
}
