#include <bits/stdc++.h>
int a[1005];
int main() {
  int n, m, h;
  while (~scanf("%d%d%d", &n, &m, &h)) {
    int sum = 0;
    for (int i = 1; i <= m; i++) {
      scanf("%d", &a[i]);
      sum += a[i];
    }
    if (sum < n) {
      printf("-1\n");
    } else {
      sum--;
      n--;
      int sum1 = sum - a[h] + 1;
      double ans = 1.0;
      for (int i = 1; i <= n; i++) {
        ans *= (sum1 * 1.0) / (sum * 1.0);
        sum1--;
        sum--;
      }
      printf("%.6lf\n", 1 - ans);
    }
  }
}
