#include <bits/stdc++.h>
using namespace std;
int a[100005];
int main(void) {
  int n, m, h, sum = 0;
  scanf("%i%i%i", &n, &m, &h);
  for (int i = 1; i <= m; i++) scanf("%i", &a[i]), sum += a[i];
  if (sum < n) return printf("-1\n"), 0;
  double ans = 1.0;
  for (int i = 1; i <= n - 1; i++) {
    ans *= (double)(sum - a[h] - n + i + 1) / (sum - n + i);
  }
  ans = 1.0 - ans;
  printf("%.6lf\n", ans);
  return 0;
}
