// stay fuckin hard -- Abhinav -IIITA
#include<bits/stdc++.h>
#define int long long
#define ld long double
using namespace std;
int mod=1e9;
int32_t main(){
	ld r; cin>>r;
	r*=2;
	ld pi=3.1415926535;
	r*=pi;
	cout<<r<<"\n";
	return 0;
}

Dr. Soumyadev Maity