#include <bits/stdc++.h>
using namespace std;
int n, m, h, i, sum, a[1001];
double s;
int main() {
  cin >> n >> m >> h;
  for (i = 1; i <= m; i++) cin >> a[i], sum += a[i];
  if (sum < n)
    printf("-1\n");
  else {
    s = 1;
    for (i = 1; i < n; i++) s = s * (sum + 1 - a[h] - i) / (sum - i);
    printf("%.10f", 1 - s);
  }
}
