#include<bits/stdc++.h>
//#include <boost/multiprecision/cpp_int.hpp>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>

#define ll long long
//#define bint int1024_t
#define ld long double
#define vect vector<ll>
#define unmap unordered_map
#define pb push_back
#define pi 3.1415926536
#define mod int(1e9+7)
#define mp make_pair
//ios_base::sync_with_stdio(false);cin.tie(NULL);
#define testcases ll t; cin>>t; while(t--)

using namespace std;
//using namespace boost::multiprecision;
//using namespace __gnu_pbds;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    // int k=0;
    // testcases
    // {
        

    //     cout<<"Case #"<<++k<<": "<<ans<<"\n";
    // }

    double r;
    cin>>r;
    cout<<setprecision(10)<<fixed<<2*pi*r;

    return 0;
}