#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, n, m, k;
  int h;
  while (scanf("%d%d%d", &n, &m, &h) != EOF) {
    int sum1, sum2;
    sum1 = sum2 = 0;
    for (i = 1; i <= m; i++) {
      scanf("%d", &k);
      if (i == h)
        sum1 = k;
      else
        sum2 += k;
    }
    k = n - 1;
    if (sum1 + sum2 < n)
      puts("-1");
    else if (sum2 < k - 1)
      puts("1");
    else {
      k = n - 1;
      h = sum2;
      double ans = 1;
      for (i = 1; i <= k; i++) {
        ans *= h;
        h--;
      }
      h = sum2 + sum1 - 1;
      for (i = 1; i <= k; i++) {
        ans /= h;
        h--;
      }
      printf("%.7lf\n", 1 - ans);
    }
  }
  return 0;
}
