shaftmine2.1

-- defining function
function Mine20()
  if turtle.detect() then
    turtle.dig()
    os.sleep(1)
  else
    for i = 1,20 do
      turtle.forward()
    end
  end
end

-- mining the lower row
Mine20()

-- move up
turtle.digUp()
turtle.turnLeft()
turtle.turnLeft()

-- mining the upper row
Mine20()
turtle.forward()