first_input = [int(x) for x in input().split(" ")]
eat_bean_numbers = [int(x) for x in input().split(" ")]

total_beans = first_input[0] * first_input[1]
total_eat_number = sum(eat_bean_numbers)

diff = total_beans - total_eat_number
res = -1 if diff < 0 else diff

print(res)