#include <iostream>
#include<cmath>
#include<algorithm>
#include<string>
#include<iomanip>
#include<string.h>
using namespace std;
// log10(x)+1  number of digits 
void bubblesort(int arr[], int n);
int main()
{
	double pi = 3.14285714285714285714;
	int r;
	cin >> r;
	cout << fixed << setprecision(20) << 2 * pi*r;
}
