double integral when using syms

Question

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syms x y a

w(1)=(x-5)*(y-5)*(x-2.5)*(y-2.5)*(4/625);

w(2)=x*(x-5)*(y-2.5)*(y-5)*(-8/625);

w(3)=x*(x-2.5)*(y-2.5)*(y-5)*(4/625);

w(4)=(x-2.5)*(x-5)*y*(y-5)*(-8/625);

w(5)=x*(x-5)*y*(y-5)*(16/625);

w(6)=x*(x-2.5)*y*(y-5)*(-8/625);

w(7)=(x-2.5)*(x-5)*(y-2.5)*y*(4/625);

w(8)=x*(x-5)*(y-2.5)*y*(-8/625);

w(9)=x*(x-2.5)*(y-2.5)*y*(4/625);

for i=1:2

    for j=1:2
        m=diff(w(i),x);
        n=diff(w(j),x);
        o=diff(w(i),y);
        p=diff(w(j),y);
        a(x,y)=m.*n+o.*p;
        fun=@(x,y)a(x,y)
        k(i,j)=integral2(fun,0,5,0,5);
    end

end

k

This is the code I am trying to run and I am getting an error "Input function must return 'double' or 'single' values. Found 'sym'." .. Any idea if this can be fixed? If not.. how do I evaluate the double integral?