#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
void solve() {
  double pi = 3.14159;
  double n;
  cin >> n;
  double a = 2;
  double ans = a * pi * n;
  cout << setprecision(6) << fixed << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
}
