nums = raw_input().split()
ages = raw_input().split()

K = int(nums[0])
L = int(nums[1])
#F = int(nums[2])
ages = map(int, ages)

ans = -1

mame_exist = K * L
mame_needed = sum(ages)

if mame_exist - mame_needed >=0 :
    ans = mame_exist - mame_needed

print ans