#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
void Yes() { cout << "Yes" << endl; }
void No() { cout << "No" << endl; }
int main(void) {
  double pi = 3.1415926535;
  double r;
  cin >> r;
  printf("%.10f\n", 2 * pi * r);
}
