#-2=ysqrt(x-y)#
#0=y/(2sqrt(x-y))+sqrt(x-y) dy/dx-y/sqrt(x-y) dy/dx#
#-y/(2sqrt(x-y))=sqrt(x-y) dy/dx-y/sqrt(x-y) dy/dx#
#-y/(2sqrt(x-y))=dy/dx(sqrt(x-y)-y/sqrt(x-y))#
#-y/(2sqrt(x-y))=dy/dx((x-y-y)/sqrt(x-y))#
#-y/(2sqrt(x-y))=dy/dx((x-2y)/sqrt(x-y))#
#(-y/(2sqrt(x-y))) *(sqrt(x-y))/(x-2y)=dy/dx#
#:. dy/dx=-y/(2x-4y)#