#include <bits/stdc++.h>
int a[2000];
int main() {
  int n, m, i, j, h, k, sum;
  double ans;
  while (scanf("%d %d %d", &n, &m, &h) != EOF) {
    sum = 0;
    ans = 1;
    for (i = 1; i <= m; ++i) {
      scanf("%d", &a[i]);
      if (i == h)
        k = a[i] - 1;
      else
        sum += a[i];
    }
    if (n > sum + k + 1) {
      printf("-1.0\n");
      continue;
    } else
      n = n - 1;
    while (sum > 0 && n > 0) {
      ans *= (double)(sum) / (double)(sum + k);
      sum--;
      n--;
    }
    if (n > 0) {
      printf("1.0\n");
      continue;
    }
    printf("%.10f\n", 1 - ans + 1e-10);
  }
  return 0;
}
