def main():
    K, N, F = map(int, input().split())
    #print(K,N,F)
    family_ages = map(int, input().split())
    #print(list(family_ages))
    beans_sum = K * N
    family_ages_sum = sum(family_ages)
    #print(family_ages_sum)
    beans_left = beans_sum - family_ages_sum
    if beans_left >= 0:
        print(beans_left)
    else:
        print(-1)


if __name__ == '__main__':
    main()