Stripmine Programm Minecraft

print "Bitte gebe ein, wie viele Blöcke die Turtle weit fahren soll:"
nextside = 2
nexttorch = 5
size = tonumber(read())
for i=0,size do
    if turtle.detect(gravel) then
        turtle.dig()
        os.sleep(0.5)
    end
    if  turtle.detect(sand) then
        turtle.dig()
        os.sleep(0.5)
    end
    if turtle.detect() then
        turtle.dig()
    end

    turtle.forward()

    if turtle.detectUp(gravel) then
        turtle.digUp()
        os.sleep(0.5)
    end
    if  turtle.detectUp(sand) then
        turtle.digUp()
        os.sleep(0.5)
    end
    if turtle.detectUp() then
        turtle.digUp()
    end
    if nextside == 0 then
        turtle.up()
        turtle.turnLeft()
        for f=0,1 do
            for j=0,4 do
                turtle.dig()
                turtle.forward()
            end
            turtle.turnLeft()
            turtle.turnLeft()
            for j=0,4 do
                turtle.dig()
                turtle.forward()
            end
        end
        turtle.down()
        turtle.turnRight()
        nextside = 3
    end
    if nexttorch == 0 then
        turtle.turnRight()
        turtle.turnRight()
        turtle.select(2)
        turtle.place()
        turtle.turnRight()
        turtle.turnRight()
        nexttorch = 6
    end
    nexttorch = nexttorch-1
    nextside = nextside-1
end