#include <bits/stdc++.h>
using namespace std;
int s[1005];
int main() {
  int n, m, h;
  scanf("%d%d%d", &n, &m, &h);
  int sum = 0;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &s[i]);
    sum += s[i];
  }
  if (sum < n) {
    printf("-1\n");
    return 0;
  }
  long double ans = 1;
  long double down = sum - 1;
  long double up = sum - s[h];
  for (int i = 1; i <= n - 1; i++) {
    ans = ans * up / down;
    up--, down--;
  }
  printf("%.20Lf\n", 1 - ans);
}
