u = ->(m) do
  y = ->(*a, &b) { y = ->(*a) { b.(*a, &y) } }

  r = ->(n, f) do
    i, g = f.()
    i <= n ? [i] + r.(n, g) : []
  end

  r.(m, y.() { |i, j, &b| -> { [j, b.(j, i + j)] } }.(1, 1))
end

p u.(4_000_000).select(&:even?).reduce(0, :+)