S = input()
# S = '00'+S
Z = []
N = len(S)
for i in range(N):
    if S[i]=='0':
        Z.append(i)
# print(Z)
ans = 0
M = len(Z)
pre = -1
for i in range(M):
    now = Z[i]
    dif = now - pre
    x = (dif - 1) // 2
    ans += x
    pre = now - x * 2
print(ans)