#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, m, h;
  cin >> n >> m >> h;
  int sum = 0, sh = 0;
  for (int i = 0; i < m; i++) {
    int x;
    cin >> x;
    sum += x;
    if (i == h - 1) sh = x;
  }
  sum--;
  sh--;
  if (sum < n - 1) {
    cout << -1;
    exit(0);
  }
  double ans = 1;
  for (int i = 0; i < sh; i++) {
    ans = ans * 1.0 * (sum - n + 1 - i) / (sum - i);
  }
  cout << fixed << 1 - ans;
  return 0;
}
