#include <bits/stdc++.h>                                     
using namespace std;                                 
                                                 
#define ll long long                           
#define MOD 1000000007                      
                                           
int main() {                             
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);     

	int n;
	cin>>n;
	double pi=3.14
	double ans=2*pi*n;
	cout<<ans<<endl;

	return 0;
}      