99*77 =7700-77 =7623
999*777 =777000-777 =776223
9999*7777 =77770000-7777 =77762223
99999*77777 =7777700000-77777 =7777622223
999999*777777 =777777000000-777777 =777776222223
9999999*7777777 =77777770000000-7777777 =77777762222223
99999999*77777777 =7777777700000000-77777777 =7777777622222223
想必規律很明顯了吧?