This is easier to judge if we see the situation when play stops:
East has 6 additional top tricks, making 8 tricks altogether, and there is no easy way to find a winning line, even double-dummy. So -1 is the correct adjust for sure.
However, if declarer wins 3 tricks in the dummy and then plays
♠, South is squeezed in the minors: he has to hold 3
♣ cards and therefore must discard one of his
♦ cards. Declarer cashes
♦A and then endplays South in
♣. He must be careful to discard !C10 in the dummy on the
♦A, otherwise South can avoid the endplay by letting the dummy win the
♣10.
Karl