The problem

The ADC, SBC, INX, and INY instructions are the only real arithmetic instructions the 6502 chip has. In and of themselves, they aren't too useful for general applications: the accumulator can only hold 8 bits, and thus can't store any value over 255. Matters get even worse when we're branching based on values; BMI and BPL hinge on the seventh (sign) bit of the result, so we can't represent any value above 127.