#include <iostream>
#include <numeric>
#include <vector>
#include <math.h>
#include <map>
#include <algorithm>
#include <stdio.h>
#include <set>
#define mod 1000000007
using namespace std; 

int main(){
	int r;
	cin >> r;
	cout << 2 * r * M_PI << "\n";
}
