#include <bits/stdc++.h>

#define pb push_back
#define f first
#define s second
#define pii pair<int, int>
#define mp make_pair
 
using namespace std;
 
const string name = "A",
             in_file = name + ".in",
             out_file = name + ".out";	
 
ifstream fin(in_file);
ofstream fout(out_file);
 

int main() {
	long long n;
	cin >> n;
	cout << 3 * (n + (n - 1)/2);
	return 0;
}