compare the triangle in polog

I’m trying to compare two triangles by two point and height.

compare_tri( triangle ( point(X1,Y1), point(X2,Y2), H1),
triangle( point(X3,Y3), point(X4,Y4), H2)) :-

A1 is ((X2-X1)*(X2-X1)),
B1 is ((Y2-Y1)*(Y2-Y1)),
C1 is (A1+B1),
D1 is (sqrt(C1)),
S1 is (D1*H1),
A2 is ((X4-X3)*(X4-X3)),
B2 is ((Y4-Y3)*(Y4-Y3)),
C2 is (A2+B2),
D2 is (sqrt(C2)),
S2 is (D2*H2),
(   (S1 < S2) ->  (S1 is 2), (S2 is 1); (S2 is 2), (S1 is 1)),
write(S1), write('bigger than '), write(S2).

but I get the error message ‘syntax operator : operator expected’

What’s the problem?