K, N, F = map(int, input().split())

A = map(int, input().split())

sum_of_mame = K * N
sum_of_age = 0

for a in A:
    sum_of_age += a

if (sum_of_mame - sum_of_age < 0):
    print(-1)
else:
    print(sum_of_mame - sum_of_age)