#include <iostream>
using namespace std;
int main()
{
float Pi=3.14;
float R;
cin>>R;
float L=2*Pi*R;
cout<<L;
return 0;
}
#include <iostream>
using namespace std;
int main()
{
float Pi=3.14;
float R;
cin>>R;
float L=2*Pi*R;
cout<<L;
return 0;
}