#include <bits/stdc++.h>
#define INF 1000000000
#define LINF 1000000000000000000
#define MOD 1000000007
#define mod 1000007
#define INF63 1061109567
#define INF127 9187201950435737471
#define UINF 18446744073709551615
#define F first
#define S second
#define ll long long
#define pi 3.141592653589793238
using namespace std;
long double r;
int main(){
	cin>>r;
	cout<<r*2*pi<<endl;
	return 0;
}