#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int sum;
int s[1005];
int main() {
  cin >> n >> m >> h;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &s[i]);
    sum += s[i];
  }
  if (sum < n)
    printf("-1\n");
  else {
    double ans = 1;
    for (int i = 0; i < n - 1; i++)
      ans *= (sum - s[h] - n + 2 + i) * 1.0 / (sum - n + 1 + i);
    ans = 1 - ans;
    cout << ans;
  }
  return 0;
}
