#include <bits/stdc++.h>
using namespace std;
int a[1005];
double zz(int n, double sum, double a) {
  double ans = 1.0;
  for (int i = 0; i < n - 1; i++) {
    ans = ans * (sum - a - i) / (sum - 1 - i);
  }
  return ans;
}
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", &a[i]);
    sum += a[i];
  }
  if (sum < n) {
    printf("-1\n");
  } else {
    if (a[h] == 1) {
      printf("0.000000\n");
    } else {
      printf("%.6f\n", 1 - zz(n, sum, a[h]));
    }
  }
}
