#include <bits/stdc++.h>
using namespace std;
bool comp(const int& a, const int& b) { return a > b; }
int main() {
  ifstream in("C:\\Users\\bachi\\source\\repos\\atcoder\\Debug\\INPUT.txt");
  cin.rdbuf(in.rdbuf());
  double r;
  cin >> r;
  cout << 2 * r * 3.14159265358979323846 << endl;
}
