#include <bits/stdc++.h>
using namespace std;
using int64 = long long;
constexpr int DEBUG = 0;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(10);
  int r;
  cin >> r;
  cout << r * 2.0 * M_PI;
}
