#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)s; i < (int)e; i++)

int main() {
    cin.tie(nullptr);
    
    cout << fixed << setprecision(5);
    
    int N;
    cin >> N;
    
    cout << 3.5 * N << '\n';
}