buffer = list(input())
S = list(map(int, buffer))
N = len(S)
i = 0
count = 0
while(1):
    if(S == [] or N == 1):
        break
    elif((S[i] + S[i + 1]) == 1):
        S.pop(i)
        S.pop(i)
        i = 0
        count += 2
        N = len(S)
    else:
        i += 1
        if(i == (N - 1)):
            break
print(count)