#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  h--;
  int sum = 0;
  int Q = 0;
  for (int i = (0); i < (m); ++i) {
    int t;
    cin >> t;
    if (i == h) Q = t;
    sum += t;
  }
  if (sum < n) {
    cout << "-1\n";
    return 0;
  }
  sum--;
  Q--;
  n--;
  double ans = 1.0;
  int it1 = sum - Q - n + 1;
  int up1 = sum - n;
  int it2 = sum - Q + 1;
  int up2 = sum;
  double eps = 1e-15;
  bool bx = true;
  while ((it1 <= up1 || it2 <= up2) && bx) {
    bx = false;
    if (ans < eps) {
      while (it1 <= up1 && ans < 1e9) {
        ans *= it1;
        it1++;
        bx = true;
      }
    }
    while (it2 <= up2 && ans > 1e-9) {
      ans = 1.0 * ans / it2;
      it2++;
      bx = true;
    }
  }
  while (it1 <= up1) {
    ans *= 1.0 * it1;
    it1++;
  }
  while (it2 <= up2) {
    ans = 1.0 * ans / it2;
    it2++;
  }
  printf("%.8f\n", 1.0 - ans);
  return 0;
}
