#include <bits/stdc++.h>
using namespace std;
int n, m, k;
int a[1001];
int sum;
double ans1 = 1;
double work() {
  int i, j;
  double ans = 1;
  for (i = 0; i < n; i++) {
    ans = ans * (sum - a[k] - i);
    ans1 = 1.0 * (sum - i);
    ans = ans / ans1;
  }
  return ans;
}
int main() {
  int i, f;
  double ans;
  cin >> n >> m >> k;
  n--;
  sum = 0;
  for (i = 1; i <= m; i++) {
    cin >> a[i];
    if (i == k) a[i]--;
    sum += a[i];
  }
  if (sum < n)
    cout << "-1" << endl;
  else {
    ans = work();
    ans = 1 - ans;
    if (ans == 1) {
      f = ans;
      cout << f << endl;
    } else
      printf("%.10lf\n", ans);
  }
  return 0;
}
