# coding: utf-8
data = list(input())
removed_block = 0
i = 1
log = data[0]
while i < (len(data) + 1):
    try:
        if data[i] != log:
            data.pop(i)
            data.pop(i - 1)
            removed_block += 2
            i = 1
            log = data[0]
        else:
            i += 1
    except:
        break
print(removed_block)
