  We construct Hadamard matrices of orders 4x251 = 1004 and 4x631 = 2524, and
skew-Hadamard matrices of orders 4x213 = 852 and 4x631 = 2524. As far as we
know, such matrices have not been constructed previously. The constructions use
the Goethals-Seidel array, suitable supplementary difference sets on a cyclic
group and a new efficient matching algorithm based on hashing techniques.
