#include<bits/stdc++.h>
#define fr(i,n) for(ll i=0;i<n;i++)
#define ll long long
#define pb push_back
#define endl "\n"
#define mod 1000000007
using namespace std;
int main()
{
	int r;
	cin>>r;
	double pi = 3.14159;
	double c=2*pi*r;
	cout << fixed << setprecision(2) << c <<endl; 
}