#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h, sh, sum = 0, s;
  scanf("%d %d %d", &n, &m, &h);
  for (int i = 1; i <= m; ++i) {
    scanf("%d", &s);
    if (i == h) sh = s;
    sum += s;
  }
  if (n > sum)
    puts("-1");
  else if (sh < 2)
    puts("0");
  else {
    n--;
    sh--;
    sum--;
    double p = 1.0;
    for (int i = 0; i < sh; ++i) p *= (double)(sum - n - i) / (double)(sum - i);
    printf("%.7lf\n", 1.0 - p);
  }
  return 0;
}
