#include <bits/stdc++.h>
using namespace std;
using P = pair<long long, long long>;
using V = vector<long long>;
const double PI = 3.141592653589;
void solve_abc_a() {
  long long r;
  float ans;
  cin >> r;
  ans = 2 * r * PI;
  cout << fixed;
  cout << setprecision(12) << ans;
  return;
}
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve_abc_a();
  return 0;
}
