Turtle w Fuel

term.write("How far shall i dig?:  ")
distance = read()
print("Digging a 1x1 blocks for "..distance.." blocks...")

 for i = 1, 16 do -- loop through the slots
  turtle.select(i) -- change to the slot
  if turtle.refuel(0) then -- if it's valid fuel
    turtle.refuel()
	end


 for i = 1, distance
 do
 turtle.dig()
 turtle.forward()
 turtle.digDown()
 turtle.digUp()
 end

end