#include <bits/stdc++.h>
using namespace std;
int n, m, h, a[1010], tot;
int main() {
  scanf("%d %d %d", &n, &m, &h);
  for (int i = 1; i <= m; i++) {
    scanf("%d", &a[i]);
    tot += a[i];
  }
  if (tot < n) {
    printf("-1\n");
    return 0;
  }
  a[h]--;
  n--;
  tot--;
  double chances = 1;
  for (int i = 0; i < n; i++) {
    chances *= max(0.0, tot - i - a[h] + 0.0) / (tot - i);
  }
  printf("%.9f\n", 1 - chances);
  return 0;
}
