#include<iostream>
#include<string>
#include<vector>
#include<algorithm>
#include<map>
#define all(x) x.begin(),x.end()
#define pb push_back
#define ll long long 
#define P pair<int,int>
#define LP pair<ll,ll>
using namespace std;
int main() {
	int n;
	cin >> n;
	if (n % 2 == 0) cout << n * 1.5 << endl;
	else cout << n * 1.5 - 0.5 << endl;
	return 0;
}