#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
const int MOD = (int)1e9+7;
const double PI = 3.14159265359;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
	double r ;
	cin>>r;
	cout<<2*PI*r;
	
	
	
	return 0;
}






























