x^2+y^2-2x-4y+5=0
x^2+y^2-2x-4y+1+4=0
x^2-2x+1+y^2-4x+4=0
(x-1)^2+(y-2)^2=0
x=1
y=2
(x-y)^1999=(-1)^1999=-1