S = input()
N = len(S)
X = [-1]
M = 0
for i, s in enumerate(S):
    if s == "0":
        X.append(i)
        M += 1
ans = 0
for i in range(1, M + 1):
    a = X[i] - X[i - 1]
    if a > 2:
        ans += (a - 1) // 2
        X[i] -= (a - 1) // 2 * 2
print(ans)