#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pi 2*acos(0.0)

int main()
{
    double r;
    cin>>r;
    cout<<setprecision(10)<<2*r*pi<<endl;
}


//    #include <chrono>
//    using namespace std::chrono;
//    auto start = high_resolution_clock::now();
//    auto stop = high_resolution_clock::now();
//    auto duration = duration_cast<milliseconds>(stop - start);
//    cout << duration.count() << endl;
