#include<bits/stdc++.h>
using namespace std;
#define PI 3.14
double circum(double r){
	double cir=2*PI*r;
	return cir;
}
int main(){
	ios base:: sync_with_stdio(false)
cin.tie(NULL);
	double r;
	cin>>r;
	cout<<circum(r);
	return 0;
}
	