#include <bits/stdc++.h>
using namespace std;
const int N = 105;
const int M = 1005;
int num[M], n, m, h;
double cal() {
  if (num[h] == 0) return 0.0;
  int sum = 0;
  for (int i = 1; i <= m; i++)
    if (i != h) sum += num[i];
  if (sum + num[h] < n) return -1.0;
  if (sum + 1 < n) return 1.0;
  double ans = 1.0;
  int all = sum + num[h];
  for (int i = 1; i < n; i++, sum--) {
    ans *= (sum * 1.0 / (all - i));
  }
  return 1.0 - ans;
}
int main() {
  scanf("%d%d%d", &n, &m, &h);
  for (int i = 1; i <= m; i++) scanf("%d", &num[i]);
  double ans = cal();
  printf("%.10f\n", ans);
  return 0;
}
