#include <bits/stdc++.h>
using namespace std;
int n, m, h, sum;
int num[1100];
int main() {
  sum = 0;
  scanf("%d%d%d", &n, &m, &h);
  for (int i = 1; i <= m; i++) {
    scanf("%d", &num[i]);
    sum += num[i];
  }
  if (sum < n) {
    printf("-1\n");
    return 0;
  }
  if (num[h] == 1) {
    printf("%.6lf\n", (double)0);
    return 0;
  }
  double ex = 1.0;
  int x = sum - num[h];
  int y = sum - 1;
  for (int i = 1; i <= n - 1; i++) {
    ex *= (x * 1.0) / y;
    x--;
    y--;
  }
  printf("%.10lf\n", (double)(1.0 - ex));
}
