#include <iostream>
#include <cmath>
#include <algorithm>
#include <bitset>
#include <iomanip>
#include <cstdio>

using namespace std;

int main() {
	double pai = 3.141592653589793;

	int64_t r;
	cin >> r;

	cout << r * 2 * pai << endl;
	return 0
}