You have an arbitrary number of model kits (which you assemble for fun and profit). Each kit comes with twenty (20) stickers, two of which are labeled "0", two are labeled "1", ..., two are labeled "9". You decide to stick a serial number on each model you assemble starting with one. What is the first number you cannot stick. You may stockpile unused numbers on already assembled models, but you may not crack open a new model to get at its stickers. You complete assembling the current model before starting the next.