#include <iostream>
using namespace std;
int main()
{
 double U = 3.14;
 int R;
cin >> R  >> U; 
 cout <<  R * 2 * U <<"\n";
}
#include <bits/stdc++.h>
using namespace std;

int main(){
    int R;
    cin >> R;
    R *= 2;
    cout << R * 3.14 << endl;
}