Multiplication of two complex numbers is ( a + ib ) ( c + id ) = ( ac - bd ) + i ( ad + bc )
x = ( ( p [ 0 ] * q [ 0 ] ) % M - ( p [ 1 ] * q [ 1 ] ) % M + M ) % M NEW_LINE y = ( ( p [ 0 ] * q [ 1 ] ) % M + ( p [ 1 ] * q [ 0 ] ) % M ) % M NEW_LINE