#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <cmath>
#include <iomanip>
#include <map>
using namespace std;
typedef long long LL;

int main(){
    LL N;
    cin >> N;
    cout << setprecision(13) << N*3.5 << endl;
    return 0;
}