#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<iomanip>
#include<cstring>
#include<map>
#include<vector>
#include<queue>
#include<utility>
using namespace std;
typedef long long int ll;

int main(){

  double n;
  cin >> n;
  cout << 3.5*n << endl;

  return 0;
}