#include<iostream>
#include<string>
using namespace std;
int main()
{
    double s,a;
    cin>>s;
    if(s>=1&&s<=1000);
    {
        a=(2*3.14159)*s;
        cout<<fixed<<setprecision(2)<<a;
    }
    else{
        cout<<"Invalid input";
    }
    return 0;
}