for six digit numbers, you may get into an infinite loop. Try 851742

875421 - 124578 = 750843
875430 - 34578 = 840852
885420 - 24588 = 860832
886320 - 23688 = 862632
866322 - 223668 = 642654
665442 - 244566 = 420876
876420 - 24678 = 851742