#include <bits/stdc++.h>
using namespace std;
int stu[1010];
double C(int n, int m) {
  double s = 1.0;
  for (int i = 1; i <= m; ++i) {
    s = s * n / i;
    n--;
  }
  return s;
}
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", &stu[i]);
    sum += stu[i];
  }
  if (sum < n)
    printf("-1.00000000\n");
  else {
    int a = sum - 1;
    int b = n - 1;
    int c = stu[h] - 1;
    double ans = 1.0 - C(a - c, b) / C(a, b);
    printf("%.8lf\n", ans);
  }
  return 0;
}
