#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1e9+7;
//const ll mod = 1e9+7;
const int inf = 0x3f3f3f3f;
const int ninf = 0xc0c0c0c0; 
int main(){
	//freopen("in.txt","r",stdin);
	double n;
	cin>>n;
	const double pi = acos(-1);
	printf("%.4lf\n" , 2*pi*n);
	return 0;
}