#include <bits/stdc++.h>
using namespace std;
int a[1001];
int main() {
  int n, m, h, i, j, sum = 0;
  long double s;
  cin >> n >> m >> h;
  n--;
  for (i = 1; i <= m; i++) {
    cin >> a[i];
    sum += a[i];
  }
  a[h]--;
  sum--;
  if (sum < n) {
    cout << "-1";
    return 0;
  } else
    s = 1;
  if (sum - a[h] < n) {
    cout << "1";
    return 0;
  } else
    for (i = 1; i <= n; i++)
      s = s * (long double)(sum - a[h] - i + 1) / ((long double)sum - i + 1);
  printf("%.6Lf", 1 - s);
  return 0;
}
