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

A = map(int, input().split())
# A = [int, input().split()]

nokori = K * N

# print(A)
# print(nokori)

for A_k in A:
    nokori -= A_k

if nokori >= 0:
    print(str(nokori))
else:
    print(str(-1))