#include <bits/stdc++.h>
using namespace std;
int main() {
  freopen("C:/Users/wyx/Desktop/Amy/wangyixuan/c++/test data/data.txt", "r",
          stdin);
  int n;
  double m;
  cin >> n;
  m = 2 * n * 3.1415926535898;
  cout << m;
  fclose(stdin);
  system("pause");
  return 0;
}
