#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>
#include <set>

using namespace std;

int main()
{	
	int n;
	cin>>n;
	double res=3.5*n;

	printf("%.16f\n",res);

	return 0;
}