#include <bits/stdc++.h>
using namespace std;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
int main() {
  cout << fixed << setprecision(10);
  int r;
  cin >> r;
  cout << 2 * 3.14159265359 * r << endl;
}
