#include <bits/stdc++.h>
using namespace std;
int n, m, h, i, sum, a[1001];
double s;
int main() {
  cin >> n >> m >> h;
  --n;
  for (i = 1; i <= m; i++) {
    cin >> a[i];
    if (h == i) a[i]--;
    sum += a[i];
  }
  if (sum < n)
    printf("-1");
  else {
    s = 1.0;
    for (i = 0; i < n; i++) s *= (double)(sum - a[h] - i) / (double)(sum - i);
    printf("%.10lf", 1 - s);
  }
  return 0;
}
