#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ul;
typedef signed long long ll;
ul over = 1000000007;

int main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed;
  ll n;
  cin >> n;
  cout << n * 7 / 2 << (n%2==0 ? "" : ".5") << endl;
  return 0;
}