S = input()
stack = []
cnt = 0

for s in S:
    if s == S[0]:
        stack.append(s)
    elif s == '0' and s != stack[-1]:
        stack.pop()
        cnt += 2
    elif s == '1' and s != stack[-1]:
        stack.pop()
        cnt += 2
    else:
        stack.sppend(s)
        
print(cnt)
         