31. Qinglong, Baihu, Zhuque, Xuanwu, are the four great divine beasts in ancient Chinese legend, each of them has treasures, the quantities of which are $a, b, c, d$ respectively. It is known that $a, b, c, d$ satisfy $a+b=30, a+c=34, a+d=32$, if the maximum value of $a+b+c+d$ is $M$, and the minimum value is $N$, then $M-N=$ . $\qquad$
$56$