/*
Grind Must Not Stop
*/
#include<bits/stdc++.h>
using namespace std;

int main()
{
    long double r;
    cin>>r;
    long double pi = 3.141592653589793238;
    cout<<setprecision(6)<<((long double)(((long double)2.000000000)*pi*r))<<endl;
    return 0;
}