#include <bits/stdc++.h>
using namespace std;
int n, m, h, sh;
long long sum;
int main() {
  int i, num;
  while (scanf("%d %d %d", &n, &m, &h) != EOF) {
    sum = 0;
    for (i = 1; i <= m; i++) {
      scanf("%d", &num);
      sum = sum + (long long)num;
      if (i == h) sh = num;
    }
    if (sum < n) {
      printf("-1.0\n");
      continue;
    }
    if (sum - sh < n - 1) {
      printf("1.0\n");
      continue;
    }
    if (sh == 1) {
      printf("0.0\n");
      continue;
    }
    double ans = 1;
    for (i = 1; i <= n - 1; i++) {
      ans *= (double)(sum - sh - (i - 1)) / (sum - i);
    }
    printf("%.16f\n", 1 - ans);
  }
  return 0;
}
