#include <bits/stdc++.h>
using namespace std;
double ans = 1;
int p, d, h;
int main() {
  cin >> p >> d >> h;
  int top = 0;
  int kh;
  for (int i = 1; i <= d; i++) {
    int x;
    cin >> x;
    top += x;
    if (i == h) {
      kh = x;
    }
  }
  if (top < p) {
    cout << -1;
    return 0;
  } else {
    double t = 1;
    int temp = top - 1;
    for (int i = p - 1; i > 0; i--) {
      t *= ((double)temp / (double)i);
      temp--;
    }
    double ist = 1;
    int temp2 = top - kh;
    for (int i = p - 1; i > 0; i--) {
      ist *= ((double)temp2 / (double)i);
      temp2--;
    }
    double aist = t - ist;
    ans = aist / t;
    cout << ans;
  }
  return 0;
}
