#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m, h, ttl = 0, ans = -1.0;
  vector<long long> p;
  cin >> n >> m >> h;
  for (long long i = 0; i < m; i++) {
    long long a;
    cin >> a;
    p.push_back(a);
    ttl += a;
  }
  if (ttl < n) {
    cout << ans << endl;
    return 0;
  }
  if (p[h - 1] == 1) {
    cout << 0.0 << endl;
    return 0;
  }
  if (ttl - p[h - 1] < n - 1) {
    cout << 1.0 << endl;
    return 0;
  }
  double bbo = ttl - 1, bsi = ttl - p[h - 1], oth = 1;
  for (long long i = 0; i < n - 1; i++) {
    oth *= ((bsi - i) / (bbo - i));
  }
  cout << 1.0 - oth << endl;
  return 0;
}
