#include <bits/stdc++.h>
#define pb push_back
#define NMAX (1<<13)
#define ll long long
#define x first
#define y second
//#define cout fout

using namespace std;

ofstream fout("fisier.in");

int main() {
	int i,j,n;

	cin>>n;
	if(n%2==0) cout<<1LL*n/2*3+1LL*n/2*6-3LL;
	else cout<<1LL*n/2*3+1LL*n/2*6+3LL;

	return 0;
}