import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

def f(s):
    n = len(s)
    # print(s)
    ans = 0
    cnt = 0
    for i in range(n):
        if s[i] == "0":
            ans += cnt // 2
            if cnt % 2:
                cnt -= 1
        else:
            cnt += 1
    return ans

s = input()
print(f(s))