a=-[(999*999-999)/(998*998+998)]
=-[999*998/998*999]
=-1
b=-[(1000*1000-1000)/(999*999+999)]
=-[1000*999/999*1000]
=-1
c=-[(1001*1001-1001)/(1000*1000+1000）]
=-[1001*1000/1000*1001]
=-1
abc=-1