#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100015;
const int MOD = 10007;
double nCr(int n, int r) {
  double p = 1.0, q = 1.0, ans = 1.0;
  if (r > n) return 0.0;
  for (int i = 0; i < r; i++) {
    p = n - i;
    q = i + 1;
    ans *= (p / q);
  }
  return ans;
}
int main() {
  int n, m, h;
  int arr[1005];
  cin >> n >> m >> h;
  int sum = 0;
  int hsum = 0;
  for (int i = 1; i <= m; ++i) {
    cin >> arr[i];
    sum += arr[i];
    if (i == h) hsum = arr[i];
  }
  if (sum < n)
    printf("%.10lf\n", -1.0);
  else {
    double num = nCr(sum - hsum, n - 1);
    double den = nCr(sum - 1, n - 1);
    printf("%.10lf\n", 1.0 - (num / den));
  }
  return 0;
}
