Question: contour plot of implicit expression

Hi,

the foolowing code is for contour plot at Delta =-12.71 but there is somthing coulnd understand not works

I dont know why this not work

restart:
with(plots):

Omega:=0:lambda:=1:
gamma1:=8*Pi:
gamma2:=0.002*Pi:
x1:=100*Pi:
omega2:=200*Pi:
gamma0:=0.2*Pi:
G:=20*Pi:

lambda1:=(1/(2*Pi))*(G*Omega*gamma0/(2*(0.25*gamma0^2+Delta^2))):
lambda2:=(1/(2*Pi))*(-G*Omega*Delta/((0.25*gamma0^2+Delta^2))):
lambda3:=(1/(2*Pi))*gamma1+lambda1:
lambda4:=(1/(2*Pi))*(lambda*Delta-G^2*Delta/(0.25*gamma0^2+Delta^2)):
lambda5:=(1/(2*Pi))*(2*x1^2*omega2/((omega2^2+gamma2^2))):
f:=epsilon=(lambda1+sqrt(-lambda2^2+Y^2*(lambda3^2+(lambda4-lambda5*Y^2)^2))):

implicitplot3d(f, epsilon=0..20,Y = 0 .. 1,Delta=-40*Pi..40*Pi, labels=[epsilon,X,Delta],tickmarks=[3,3,3],style=surface);

contourplot(solve(f,Delta),epsilon=-20..20,Y=0..1,contours=[-12.71],axes=boxed,thickness=2,color=black,font=[1,1,18],tickmarks=[2, 2],linestyle=1, numpoints=50000);