999*1999
=(1000-1)(2000-1)
=2000000-3000+1
=1997000+1
=1997001
824-396
=824-(400-4)
=424+4
=428
763+298
=763+300-2
=1063-2
=1061
9999*7778+3333*6666
=3333(3*7778+6666)
=9999(7778+2222)
=9999*10000
=99990000
1234567891^2-1234567890*1234567892
=1234567891^2-(1234567891-1)(1234567891+1)
=1234567891^2-(1234567891^2-1^2)
=1