#include<iostream>
#include<algorithm>
#include<stdio.h>
#include<stdlib.h>
#include<vector>
#include<string>
#include<sstream>
#include<math.h>
using namespace std;


int main(){

  long long int n;
  cin >> n;

  printf("%.4lf\n",3.5*n);

  return 0;
}