#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
const int N = 200001;
void solve() {
  int n;
  cin >> n;
  double ans = 2 * 22 * n;
  ans = ans / 7;
  cout << fixed << setprecision(10) << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int testcase = 1;
  while (testcase--) {
    solve();
    cout << '\n';
  }
  return 0;
}
