#include <bits/stdc++.h>
using namespace std;
int a[1005];
int main() {
  int sum1, h, n, m, sum2;
  while (~scanf("%d%d%d", &n, &m, &h)) {
    sum1 = sum2 = 0;
    for (int i = (1); i <= (m); ++i) {
      scanf("%d", &a[i]);
      sum1 += a[i];
      if (i != h) sum2 += a[i];
    }
    if (sum1 < n) {
      printf("-1\n");
      continue;
    }
    if (sum2 + 1 < n) {
      printf("1.0\n");
      continue;
    }
    double ans = 1.0;
    for (int i = 0; i < (n - 1); ++i) {
      ans = ans * (sum2 - i) / (sum1 - 1 - i);
    }
    printf("%.8lf\n", 1 - ans);
  }
  return 0;
}
