#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll MOD = 1000000007;
const double PI = 3.14159265358979;
const ll INF = pow(10, 18);
string abc = "abcdefghijklmnopqrstuvwxyz";
string ABC = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(10);
  cout << 2 * r * PI << endl;
}
