from collections import deque
r=deque(map(int,list(input())))
l=deque()
cnt=0
while len(r)>0:
	if len(l)==0:
		l.append(r.popleft())
	if len(r)==0:
		break
	if l[-1] + r[0]==1:
		l.pop()
		r.popleft()
		cnt += 2
	else:
		l.append(r.popleft())
	print(l,r)
		
print(cnt)