#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h, suma, sh, i, j, a;
  long long int res;
  double prob;
  cin >> n >> m >> h;
  suma = 0;
  for (i = (0); i < (m); i++) {
    cin >> a;
    if (i + 1 == h) {
      sh = a;
    }
    suma += a;
  }
  prob = 0.0;
  if (suma > 1 && suma >= n) {
    if (suma - sh < n - 1) {
      prob = 1.0;
    } else {
      prob = 1.0;
      res = 1LL;
      i = suma - n + 1;
      j = suma - sh - n + 2;
      while (i <= suma - 1 && j <= suma - sh) {
        prob *= double(i) / double(j);
        i++;
        j++;
      }
      while (i <= suma - 1) {
        prob *= double(i);
        i++;
      }
      while (j <= suma - sh) {
        prob /= double(j);
        j++;
      }
      prob = 1.0 - 1.0 / double(prob);
    }
  }
  if (suma < n) {
    prob = -1.0;
  }
  cout << prob << endl;
  return 0;
}
