S = input() + "00"
N = len(S)
f = 0
ans = 0
for i in range(N):
    if f == 0:
        if S[i] == "1":
            f = 1
            cnt1 = 1
            cnt2 = 0
    elif f == 1:
        if S[i] == "1":
            cnt1 += 1
        else:
            if cnt1 == 1:
                ans += 1
                f = 0
            else:
                cnt2 = 1
                ans += 1
                f = 2
    elif f == 2:
        if S[i] == "1":
            cnt2 = 0
        else:
            cnt2 += 1
            if cnt2 == 1:
                ans += 1
            else:
                f = 0
                ans += 1
    #print(i,S[i],ans,f,cnt1,cnt2)
print(ans)