ComputerCraft turtle program - mining layer

local usedFuel = 0

function fuelChk()
    if turtle.getFuelLevel() < 20 then
        turtle.select(1)
        if turtle.refuel(1) == false then
            print("Out of fuel!")
            exit()
        end
        usedFuel = usedFuel + 1
    end
end

function doStuffAndThings()
    print("Thank you for using our amazing freaking program")
    print("Q009 says hello")
    print("Please specify length of the area")
    local len = io.read()
    print("Please specify width of the area")
    local width = io.read()
    len = tonumber(len)
    width = tonumber(width)
    local foundMats = 0
    if len < 4 then len = 4 end
    if width < 4 then width = 4 end

    fuelChk()
    while turtle.detect() do
        turtle.dig()
        sleep(0.5)
    end

    turtle.forward()
    for i = 1, len, 1 do
        fuelChk()
        if i % 2 == 0 then turtle.turnRight()
        else turtle.turnLeft() end
        for j = 1, width, 1 do
            turtle.select(2)
            if turtle.detectDown() and turtle.compareDown()
            then
                turtle.digDown()
                foundMats = foundMats + 1
            end

            while (turtle.detect()) and (j < width) do
                turtle.dig()
                sleep(0.5)
            end
            if j < width then turtle.forward() end
        end

        if i % 2 == 0 then turtle.turnLeft()
        else turtle.turnRight() end
        while turtle.detect() do
            turtle.dig()
            sleep(0.5)
        end
        turtle.forward()
    end

    turtle.back()
    if len % 2 ~= 0 then
        turtle.turnRight()
        for i = 1, width, 1 do turtle.forward() end
        turtle.turnLeft()
    end
    turtle.turnLeft() turtle.turnLeft()
    for i = 1, len, 1 do turtle.forward() end
    print("Done! Used fuel: " .. usedFuel .. " | ammount mined material: " .. foundMats)
end

doStuffAndThings()