#include <bits/stdc++.h>
#include <vector>
#include <cmath>


using namespace std;


int main()
{
     int R;
    
    cin>>R;
    
    
    float result = 2*M_PI*R;
    
    cout<<result;
    
     
    
    return 0;
}#include <bits/stdc++.h>
#include <vector>
#include <cmath>


using namespace std;


int main()
{
     int R;
    
    cin>>R;
    
    
    float result = 2*M_PI*R;
    
    cout<<result;

    return 0;
}