#include <bits/stdc++.h> 
using namespace std;
const int MOD = 1000000007;
int main() 
{ 
  
   // print circumferences of a circle of R radius. formula C = 2 * pi * r;
    int r;
    cin >> r;
    float pi = 3.14159265359;
    float ans = (float)(2*pi*r);
     printf("%0.20f", ans);

    return 0; 
}