# Finding Cremona label for Tunnell's y^2-x^3-n^2x

When I enter

E = EllipticCurve([-n^2,0]); E;E.label()

Sage gives an error that I have an unsupported operand type. I assume the problem is that I have not defined the variable 'n', which is a square free integer, but how does one do this? Any help would be greatly appreciated!!