100<= 39a+b <=999
0<=b<=38
0<= 40b <=1520
-1520<= -40b <=0
-1420<=(39a+b-40b)<=999
-1420<=39(a-b)<=999
-36.4<=(a-b)<=25.6
ab都为整数,a-b也是整数,为[-36,25]之间的所有可能的整数