S = input()

IDX = []
for i in range(len(S)):
    if S[i] == "0":
        IDX.append(i)

ans = 0
A = [-1]
for idx in IDX:
    diff = idx-A[-1]-1
    ans += diff//2
    if (A[-1]+1)%2 == idx%2:
        A.append(A[-1]+1)
    else:
        A.append(A[-1]+2)

print(ans)