#include<iostream>
#include<algorithm>
#include<cmath>

using namespace std;
typedef long long ll;
const ll limit = 1e6+2;
long long a[limit];
long long n,m,d,temp,op;
float tp,t,mp=2;

int main()
{
    cin>>t;t*=2;
    tp=6.28318530717958623200/mp*t;
    printf("%0.20f",tp);
}
