#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define l long
#define pb push_back
#define mp make_pair
#define mt make_tuple
int main() {
	// your code goes here
	//ifstream cin("input.txt");
	//ofstream cout("output.txt");
	ios_base::sync_with_stdio(false); 
    cin.tie(NULL); 
	cout<<fixed<<setprecision(20);
	double r; cin>>r;
	double ans = 2*3.14159265358979323846*r;
	cout<<ans;
	
	return 0;
}
