S = input()
 
result = 0
while True:
    if '01' in S:
        index = S.find('01')
        S = S[:index] + S[index+2:]
        result += 2
    elif '10' in S:
        index = S.find('10')
        S = S[:index] + S[index+2:]
        result += 2
 
    else:
        break
 
print(result)