#include <bits/stdc++.h>
using namespace std;
int in[1005];
int main() {
  int n, m, h, s = 0;
  scanf("%d %d %d", &n, &m, &h);
  for (int i = 1; i <= m; ++i) {
    scanf("%d", in + i);
    s += in[i];
  }
  if (s < n) return !printf("-1\n");
  double ans = 1;
  for (int i = 1; i < n; ++i) {
    ans = ans * (s - in[h] + 1 - i) / (s - i);
  }
  printf("%.10f\n", 1 - ans);
}
