nof_mame_K, nof_fukuro_N, nof_family_F = map(int, input().rstrip().split(' '))
family_ages = list(map(int, input().rstrip().split(' ')))

rest_mame = (nof_mame_K * nof_fukuro_N) - sum(family_ages)

if rest_mame < 0:
    rest_mame = -1

print(rest_mame)