#include <bits/stdc++.h>

#define ll long long
#define ld long double
#define rep(i,a,b) for (int i = a; i < b; i++)
#define irep(i,a,b) for (int i = a; i > b; i--)
#define print(n) cout << n << endl
#define rup(a,b) (a+b-1)/b

using namespace std;

int main(){
    cout << fixed << setprecision(15);
    
    int n;cin>>n;
    print(3.5*n);
    
    //system("pause");
    return 0;
}