# summer vacations, number of assignments
N, M = list(map(int, input().split()))
# each entry in A represents number of days to do the assignment
A = list(map(int, input().split()))

number_of_days_required = sum(A)
maximum_hangout_days = N - number_of_days_required
if maximum_hangout_days >= 0:
    print(maximum_hangout_days)
else:
    print(-1)