#include <bits/stdc++.h>
int n, m, h;
int s[1001] = {0};
double solve(int n, int sum) {
  if (n == 0) return 0;
  double res = 0;
  res += 1.0 * s[h] / sum;
  res += 1.0 * (sum - s[h]) / sum * solve(n - 1, sum - 1);
  return res;
}
int main() {
  scanf("%d%d%d", &n, &m, &h);
  int i;
  int sum = 0;
  for (i = 0; i < m; i++) {
    scanf("%d", &s[i]);
    sum += s[i];
  }
  if (sum < n) {
    printf("%.10lf", -1.0);
    return 0;
  }
  h--;
  s[h]--;
  n--;
  sum--;
  printf("%.10lf", solve(n, sum));
  return 0;
}
