#include <bits/stdc++.h>
using namespace std;
const int Maxn = 1000 + 5;
int n, sum, m, h, arr[Maxn];
int main() {
  cin >> n >> m >> h;
  n--;
  h--;
  for (int i = 0; i < m; i++) cin >> arr[i], sum += arr[i];
  sum--;
  if (sum < n) {
    cout << -1 << endl;
    return 0;
  }
  arr[h]--;
  long double ans = 1;
  for (int i = sum - arr[h] - n + 1; i <= sum - n; i++) {
    ans *= (long double)i;
    ans /= (long double)(i + n);
  }
  cout << setprecision(6) << fixed << 1 - ans << endl;
  return 0;
}
