S = input()

nums = []
for s in S:
    nums.append(int(s))

count = 0
i = 0
while i < len(nums)-1:
    if nums[i] != nums[i+1]:
        nums.pop(i+1)
        nums.pop(i)
        count += 1
        i -= 1
        continue
    i += 1

print(count*2)
