#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define hell  1000000007
#define ios ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define pb   push_back

int main()
{
	ios;
	ll n;
	cin>>n;
	cout<<2*n*3.141592653589793238;
}