def fibonacci_nums_up_to(n)
  result = [1] of Int32

  a = 1
  b = 2
  accum = 0

  while accum < n
    accum = a + b
    result << b
    a = b
    b = accum
  end

  result
end

puts fibonacci_nums_up_to(4000000).select{ |n| n.even? }.sum