#include <bits/stdc++.h>
using namespace std;
int n, m, h, a[1005];
double pro, bbb, ccc;
int main() {
  scanf("%d%d%d", &n, &m, &h);
  int tot = 0;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &a[i]);
    tot += a[i];
  }
  if (tot < n) {
    printf("-1.0\n");
    return 0;
  }
  a[h]--;
  pro = 1.0;
  for (int i = 1; i < n; i++) {
    tot--;
    bbb = tot - a[h];
    ccc = tot;
    bbb = bbb / ccc;
    pro = pro * bbb;
  }
  printf("%lf\n", 1 - pro);
  return 0;
}
