#include <bits/stdc++.h>
int a[2000];
int main() {
  int m, n, k, i, j;
  while (~scanf("%d%d%d", &n, &m, &k)) {
    int s = 0;
    for (i = 1; i <= m; i++) {
      scanf("%d", &a[i]);
      s += a[i];
    }
    if (s < n) {
      printf("-1\n");
      continue;
    }
    double ans, tmp = 1.0;
    int s1, s2;
    s1 = s - 1;
    s2 = s - a[k];
    for (i = 1; i <= n - 1; i++) {
      tmp = tmp * s2 / s1;
      s2--;
      s1--;
    }
    ans = 1.0 - tmp;
    printf("%.15f\n", ans);
  }
  return 0;
}
