s = list(map(int, list(input())))

count = 0
while(True):
    old = count
    if (len(s) == 1 or len(s) == 0):
        break
    for n in range(len(s) - 1):

        if (s[n] != s[n+1]):

            s.pop(n)
            s.pop(n)
            count += 2
            n -= 1

        if (len(s) <= n + 2):
            break
    if (old == count):
        break

print(count)
