hi, i have a question

consider function f(x,y) = 1 - x^2 - y^2

sketch level curves for f(x,y) = c for c = -5,0,1

that is what i am to do....is it possible to sketch for c = 1? because when i re-arrange to get y in terms of everything i get y = sqroot ( (1-c)/3 - (2x^2)/3 )

which means if c = 1 we cant graph the solution, am i correct?