#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m, d;
  cin >> n >> m >> d;
  d--;
  long long arr[m], sum = 0;
  for (int i = 0; i < m; i++) {
    cin >> arr[i];
    sum += arr[i];
  }
  if (sum < n) {
    cout << -1 << endl;
    return 0;
  }
  if (sum == n and arr[d] > 1) {
    cout << "1.000000";
    return 0;
  }
  double ans = 1.0;
  for (int i = 0; i < n - 1; i++) {
    ans *= ((sum - arr[d] - i) / (double)(sum - i - 1));
  }
  printf("%0.6f", 1 - ans);
  return 0;
}
