s=list(input())
t=[]
a=0
c=0
for i in s:
	if i=='1':
		c+=1 
	else:
		if c%2==0:
			a+=c 
			c=0 
		else:
			a+=c-1 
			c=0
			t.append('1')
		t.append('0')
for i in range(a+c):
	t.append('1')
x=0
y=0
for i in range(len(s)):
    if s[i]=='0':
        x+=i 
    if t[i]=='0':
        y+=i 
z=abs(x-y)
print(z//2)