#include <iostream>
#include<string>
#include<set>
#include<map>
#include<vector>
using namespace std;

int main()
{
    int n;
    cin>>n;
    int s[5]={0, 3, 6, 12, 15};
    long long int sol=0;
    if(n<=4)
        cout<<s[n];
    else{
        if(n%2==0){
            sol = 12+((n-6)/2)*3 + 2*(6+((n-6)/2)*3);

        }
        else{
            sol = 9 + ((n-5)/2)*3 + 2*(6+((n-5)/2)*3);
        }
 cout<<sol;

    }


    return 0;
}