#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000

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