#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m, h, sumy, i, x, y;
  scanf("%lld%lld%lld", &n, &m, &h);
  sumy = 0;
  for (i = 1; i <= m; i++) {
    if (i == h) {
      scanf("%lld", &x);
    } else {
      scanf("%lld", &y);
      sumy += y;
    }
  }
  x--;
  n--;
  if (n > x + sumy) {
    printf("-1");
    return 0;
  }
  if (x == 0) {
    printf("0.0\n");
    return 0;
  }
  if (sumy < n) {
    printf("1.0\n");
    return 0;
  }
  long long a = sumy;
  long long b = sumy + x;
  double rate = 1;
  while (a > 0) {
    rate = rate * a;
    rate = rate / b;
    a--;
    b--;
  }
  a = sumy + x - n;
  while (b > 0) {
    rate = rate * a;
    rate = rate / b;
    a--;
    b--;
  }
  b = sumy - n;
  while (a > 0) {
    rate = rate * a;
    rate = rate / b;
    a--;
    b--;
  }
  printf("%.10lf", 1 - rate);
  return 0;
}
