#include <stdio.h>
#include <iostream>
#include <map>
#include <vector>
#include <string>

#define _USE_MATH_DEFINES
#include<math.h>


using namespace std;

using ll = long long;
constexpr ll N = 2e5;
ll a[N];

int main()
{
	int R;
	cin >> R;
	
	cout << 2 * M_PI * R << endl;
	return 0;
}