Okey my problem is

if  (x2-x1) > (y2-y1)

then  dmax=x2-x1

else  dmax=y2-y1

I try with

dmax>= x2-x1

dmax>=y2-y1

but dmax not save the value more big , for example

dmax>= 50 -40

dmax>= 30 -10

the value than is obtained :

dmax = 60

but the correct value should be  dmax= 20