#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const long long int M1 = 998244353;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int n, m, h;
  cin >> n >> m >> h;
  long double sum = 0;
  vector<long double> v(m);
  for (long long int i = 0; i < m; i++) {
    cin >> v[i];
    sum += v[i];
  }
  h--;
  sum -= v[h];
  long double res = 1.0;
  for (long long int i = 1; i < v[h]; i++) {
    res *= (long double)(sum - n + 1 + i) / ((long double)(sum + i));
  }
  if (sum + v[h] < n)
    cout << -1 << '\n';
  else if (sum + 1 < n)
    cout << 1 << '\n';
  else {
    cout << 1 - res << '\n';
  }
}
