#include <bits/stdc++.h>
int main() {
  int i, j, k, n, t;
  int m, dept[1008], h, sum;
  scanf("%d%d%d", &n, &m, &h);
  sum = 0;
  for (i = 0; i < m; i++) {
    scanf("%d", &dept[i]);
    sum += dept[i];
  }
  double prob = 1.000000000;
  if (sum < n)
    printf("-1.0\n");
  else if (dept[h - 1] == 1)
    printf("0.0\n");
  else {
    dept[h - 1]--;
    n--;
    sum--;
    for (i = 0; i < dept[h - 1]; i++) {
      prob *= (sum - n - i);
      prob /= (sum - i);
    }
    {
      prob = 1.0 - prob;
      printf("%0.12lf", prob);
      printf("\n");
    }
  }
  return 0;
}
