jsa15

function digIt()
  while turtle.detect() do
    turtle.dig()
    os.sleep(0.5)
  end
  turtle.forward()
  while turtle.detectDown() or turtle.detectUp() do
    turtle.digUp()
    turtle.digDown()
  end
end

function placeTorch()
  turtle.select(16)
  turtle.placeUp()
end

local run = 0
local j = 0
local k = 0

term.write("Tunnel length: ")
run = read()

for i = 1, run do
  k = i - 1
  j = k % 4
  digIt()
  turtle.turnLeft()
  digIt()
  turtle.turnRight()
  turtle.turnRight()
  turtle.forward()
  digIt()
  turtle.back()
  turtle.turnLeft()
  if i == 2 or j == 1 then
    turtle.back()
    turtle.turnRight()
    turtle.forward()
    placeTorch()
    turtle.back()
    turtle.back()
    placeTorch()
    turtle.forward()
    turtle.turnLeft()
    turtle.forward()
  end
end


turtle.down()
turtle.turnRight()
turtle.turnRight()
turtle.select(16)
turtle.placeUp()

for i = 1, run do
  turtle.forward()
end