S=input()
N=len(S)
v=[0]*N
h=[]
for i in range(N):
  if S[i]=='0':
    h.append(i)
v=[0]*N
for i in range(N):
  if S[i]=='1':
    v[i]=v[i-1]+1
  else:
    v[i]=v[i-1]
result=0
a=0
for i in range(len(h)):
  pos=h[i]
  if i==0:
    b=v[pos]
  else:
    b=a+h[i]-h[i-1]-1
  result+=b//2
  if b%2==0:
    a=b
  else:
    a=b-1
print(result)