import collections
i,j = 0,0 
q = collections.deque([])
while(j < len(s)):
	while(s[j] == s[i]): j+=1
	q.append(j - i - 1)
	j = i

#5 6 5 1 1 4 2 1 1 1 1 1
ns = len(s)
while(len(q) > 1):
	u,v = q.popleft(), q.popleft()
	if q: q[0] += abs(u-v)
print ns - (q[0] if q else 0)
