#include <bits/stdc++.h>
using namespace std;

int main()
{
    ios::sync_with_stdio();cin.tie(0);cout.tie(0);
    #define pb push_back
    #define mk make_pair
    //#define int long long int
    float r;
    cin>>r;
    cout<<setprecision(20)<<fixed<<2*r*3.141592653589793238<<endl;



    return 0;
}
