#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
ll N,K;
int main(){
	ios_base::sync_with_stdio(0);cin.tie(0);
	cin >> N;
	N--;
	if(N%2) cout << (N/2+1)*6+(N/2)*3;
	else cout << (N/2)*6+(N/2+1)*3; 
}