S=input()

now=0
ANS=0

for i in range(1,len(S)):
    #print(now)
    if now<=1 and S[i]==S[i-1]=="1":
        now=2
        continue

    if now>=2 and S[i]=="1":
        now+=1

    if now>=2 and S[i]=="0":
        now=now//2*2
        ANS+=now//2

print(ANS)