#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int s[1007];
int main() {
  while (~scanf("%d%d%d", &n, &m, &h)) {
    int sum = 0;
    for (int i = 0; i < m; i++) {
      scanf("%d", &s[i]);
      sum += s[i];
    }
    n--;
    sum--;
    if (sum < n) {
      puts("-1");
      continue;
    }
    int sum2 = sum - (s[h - 1] - 1);
    double ans = 1;
    for (int i = 0; i < n; i++) {
      ans *= sum2 * 1.0 / (sum * 1.0);
      sum2--;
      sum--;
    }
    printf("%.7f\n", 1.0 - ans);
  }
}
