#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e18 + 5;
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long r;
  cin >> r;
  double pi = 3.14159265359;
  double ans = 2 * pi * r;
  cout << fixed << setprecision(20) << ans << '\n';
  return 0;
}
