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

double const M_PI=3.14159265358979323846;

int main()
{
    //freopen("IN.txt","r",stdin);
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    double r;
    cin>>r;
    cout<<2*M_PI*r;
    return 0;
}