#pragma GCC optimize("Ofast")
#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<math.h>
#include<map>
#include<iomanip>
#include<queue>

const long long INF_L = 1e17+7;
const long long INF_I = 1e9+7;
const long long MOD = 1e9+7;
const double EPS = 1e-8;
const double PI=acos(-1);

using namespace std;

int main(){	
	
	long long n;
	cin >> n;
	long double ans=(double)(n)*(3.5);
	//cout << fixed << setprecision(10) << ans << endl;
	cout << ans << endl;
	
	return 0;
}