#include<iostream> 
#include<algorithm> 
#include<vector> 
using namespace std; 
#define PI acos(-1)
//const double PI =  3.1415926;
int main() 
{ 
	int n;
	cin>>n;
	//cout<<2*n*PI<<"\n";
	printf("%.20lf\n",2*n*PI);
} 
