#include <iostream>
#include <iomanip>

#include <algorithm>
#include <string>
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////

int main(void){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed;//
	//cout << setprecision(7);//
	
	int N;
	cin>>N;//[1,1000000=10^6]

	P(3.5*N);

	return 0;
}