#include <bits/stdc++.h>
using namespace std;
int h[1010];
int main() {
  int n, m, k, s = 0, a;
  cin >> n >> m >> k;
  k--;
  for (int i = 0; i < m; i++) {
    cin >> h[i];
    s += h[i];
  }
  long double res;
  if (s < n)
    cout << "-1.0" << endl;
  else {
    n--;
    if (n > s - h[k])
      res = 0;
    else {
      res = 1;
      a = s - n - 1;
      if (!a) a = 1;
      s--;
      for (int i = 0; i < h[k] - 1; i++) res *= (long double)(a - i) / (s - i);
    }
    cout << fixed << setprecision(12) << 1 - res << endl;
  }
  return 0;
}
