#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<string>
#include<cstring>
#include<algorithm>
#include<queue>
#include<map>
#include<set>
typedef long long ll;
const ll longinf=1LL<<60;
using namespace std;


int main()
{
    float r;
    cin>>r;
    cout<<2*M_PI*r<<endl;
    return 0;
}

