#include <bits/stdc++.h>
using namespace std;
double a, b, gcd, n1, n2, n3, n4, cnt1, cnt2, maxi, mini, cnt, ans1, ans2, n,
    x[200000 + 5], room, p;
double tmp1, tmp2;
double c(double tp1, double tp2, double down) {
  double ret = 1;
  for (int i = 0; i < down; i++) {
    ret *= (tp1 - i) / (tp2 - i);
  }
  return ret;
}
int main() {
  cin >> n >> room >> p;
  double sum = 0;
  for (int i = 0; i < room; i++) {
    cin >> x[i];
    if (i == p - 1) a = x[i];
    sum += x[i];
  }
  double ans = c(sum - a, sum - 1, n - 1);
  if (sum < n)
    printf("%d\n", -1);
  else
    printf("%.7f\n", 1 - ans);
}
