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

int main()
{
    //freopen("a.txt","r",stdin);
    double r,area, pi=3.14159;
    cin>>r;
    area=2*pi*r;
    cout<<area;
    //printf("%lf\n",area);

}
