#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int sumall = 0;
int sumother = 0;
int sumsame = 0;
int main() {
  scanf("%d %d %d", &n, &m, &h);
  int in;
  for (int i = 0; i < m; i++) {
    scanf("%d", &in);
    sumall += in;
    if (i == h - 1) {
      sumsame = in - 1;
    } else {
      sumother += in;
    }
  }
  if (sumall < n) {
    printf("-1.0\n");
    return 0;
  }
  if (sumsame == 0) {
    printf("0.0\n");
    return 0;
  } else {
    if (sumall == n) {
      printf("1.0\n");
      return 0;
    }
  }
  if (sumother < n - 1) {
    printf("1.0\n");
    return 0;
  }
  double sol = 1;
  int p = sumother, q = sumall - 1;
  for (int i = 0; i < n - 1; i++) {
    sol *= ((double)p / (double)q);
    p--;
    q--;
  }
  printf("%.9lf\n", (double)1.0 - sol);
  return 0;
}
