#include<bits/stdc++.h>

using namespace std;

#define ll             long long 
#define mod            1000000007
#define FIO            ios_base::sync_with_stdio(false); cin.tie(NULL);
                  
void initial()
{
	#ifndef ONLINE_JUDGE
	freopen("input.txt","r",stdin);
	freopen("output.txt","w",stdout);
	#endif
}


int main() 	 	
{
	initial();
	FIO;

	long double r;
	cin>>r;

	long double circum = 2*3.14*r;

	cout<<fixed<<setprecision(20)<<circum<<endl;
}

