S(和)=a+b+19
①=S-a-13=b+6
②=S-M-①=a+13-M
③=S-M-b=a+19-M
④=b+M+13
M+a+④=S
M=16
如不明白在咨询.
M 1 2
b a 19
3 13 4
懂了吧?!