#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

void solve() {
  int N;
  cin >> N;
  cout << double(N * 3.5) << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(3);
  solve();
  getchar();
}