attempt to compare string with number expected, got string

Hi, I'm begining in lua and I made a program to automaticaly have obsidian, and put it in a chest.
Here's the code :

print("How many obsidian ?")
local nombreDemande = read()	-- read the number of obsidian requested
local nombreMine = 0	-- number of mined obsidian
term.clear()
term.setCursorPos(1,1)
print("Commande : "..nombreDemande)

while nombreMine <= nombreDemande do

if turtle.getFuelLevel() < 10 then
  turtle.select(1)
  turtle.refuel(1)
end
  turtle.select(2)	-- Placing redstone
  turtle.forward()
  turtle.forward()
  turtle.place()
  turtle.back()
  turtle.place()
  turtle.back()
  turtle.place()

  rs.setOutput("left", true)	-- Desactivate piston and lava goes on redstone
  sleep(8)  
  rs.setOutput("left", false)	-- Reactivate piston
  sleep(2)

  if turtle.getFuelLevel() < 10 then
	turtle.select(1)
	turtle.refuel(1)
  end

  turtle.dig()  -- Mine the obsidian
  nombreMine = nombreMine + 1
  turtle.forward()
  turtle.dig()
  nombreMine = nombreMine + 1
  turtle.forward()
  turtle.dig()
  nombreMine = nombreMine + 1

  turtle.back()	-- Return on the default place
  turtle.back()
  turtle.select(3)	-- Put the obsidian in the chest
  turtle.turnRight()
  turtle.drop()
  turtle.turnLeft()
end

print("Here are your obsidians")

If it don't works, here's pastebin : http://pastebin.com/7QKCeCtr

Edit : I forgot it. When I put the number of obsidian I'm looking for, the error appears :

untitled:9: attempt to compare string with number expected, got string

Sorry for my english, I'm french
Thanks for your help
floguaz

Split into new topic.

Use tonumber(read()).

And for the name of the variable ? What does "tonumber" ?

Thanks
floguaz

tonumber translates the string you get from read() to a number so you could put it inside a variable like so
variable = tonumber(read())

Edit: yay 500th post

Ok thanks ! It works !
At the begining of my program, it has worked, so why now it does that ?