#include<bits/stdc++.h>
using namespace std;
#define what_is(x) cerr<<#x <<" is "<<x<<endl;
#define ll long long 
#define gcd(a,b) __gcd(a,b)
#define pi 2*acos(0.0)
//use emplace_back() instead of push_back()
template<typename T>
void print(T t)
{
	for(const auto& e:t)cout<<e<<" ";
	cout<<endl;
}
int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	
	ll t,n,k,r;
	cin>>r;
	cout<<(double)2*3.14159265358*r<<"\n";
	return 0;
}