#include <bits/stdc++.h>
using namespace std;
#define int long long int
#define PI 3.141592653
  
float circumference(double r) 
{ 
    float cir = 2*PI*r; 
    return cir; 
} 
  
int32_t main() 
{
      double r;
      cin>>r;
      cout<<circumference(r);
      
	return 0;
}
