
#include<bits/stdc++.h>
#include<iostream>
#include<string.h>
#include<math.h>
using namespace std;
int main()
{
    float a,b[10000],c=0,e,d,i,j=0;
    cin>>a;
    e=6.28318530717958623200*a;
    cout<<e<<endl;

}
