K, N, F = map(int, input().split())
family = list(map(int, input().split()))
num_sum = N * K
age_sum = 0

for i in family:
    age_sum = age_sum + i
if num_sum >= age_sum:
    left = num_sum - age_sum
    print(left)
elif num_sum < age_sum:
    print('-1')