#include<cstdio>
#include<iostream>
#include<bits/stdc++.h>
#include<vector>
#include<algorithm>
#include<string>
#include<cmath>
#include<iomanip>
using namespace std;

int main(){

        ios::sync_with_stdio(false);
        cin.tie(0);
	
	int n;
	cin >> n;
	
	double ans= 2*M_PI*n;
	cout<<setprecision(15)<<ans<<"\n";
	


	return 0;
}
