#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int a[2005];
double ans;
int main() {
  scanf("%d%d%d", &n, &m, &h);
  int sum = 0;
  for (int i = 1; i <= m; ++i) {
    scanf("%d", &a[i]);
    sum += a[i];
  }
  ans = 1;
  for (int i = 1; i < n; ++i) {
    ans *= 1.0 * (sum - a[h] - i + 1) / (sum - i);
  }
  if (sum < n) ans = 2;
  printf("%.15lf", 1 - ans);
  return 0;
}
