------Tunneling Automaton - V2------

----Variables----

t = turtle

----Basic Controls----

function goForward()
    if t.detect() then
        t.digForward()
        t.forward()
    else
        t.forward()
    end
end
function goDown()
    if t.detectDown() then
        t.digDown()
        t.down()
    else
        t.down()
    end
end
function goUp()
    if t.detectUp() then
        t.digUp()
        t.up()
    else
        t.up()
    end
end

function refuel()
    t.select(1)
    t.refuel()
end

function length()
    num = io.read()
    num = tonumber(num)
    return num
end

----Bridge----

function layers()
    num = length()
    copynum = num
    while num > 0 do
        goDown()
        nut = 4
        while nut > 0 do
            goForward()
            goForward()
            t.turnLeft()
            nut = nut - 1
        end
        num = num - 1
    end
    goForward()
    t.turnLeft()
    goForward()
    while copynum > 0 do
        goUp()
    end
end

----Main----
function main()
    refuel()
    layers()
end

--Run--
main()