S = input()
#print(S)
ans = 0

S = list(S)

for i in range(len(S) - 1):
    if S[i] == S[i + 1] == "1":
        S = S[i:]
        break
else:
    print(0)
    exit()

while S[-1] == "1":
    S.pop()
    
    if not len(S):
        break

if len(S) == 0 or len(S) == 1:
    print(0)
    exit()

if len(set(S)) == 1:
    print(0)
    exit()

tmp = []

#print(S)

streak = 1
for i in range(1, len(S)):
    if S[i] == S[i - 1]:
        streak += 1
    else:
        tmp.append(streak)
        streak = 1
    
tmp.append(streak)

A = []
B = []

for i in range(len(tmp)):
    if i % 2 == 0:
        A.append(tmp[i])
    else:
        B.append(tmp[i])

x = A[0]
if x % 2 == 1:
    x -= 1
    
for i in range(len(A)):
    if i != 0:
        x += A[i]
        
        if x % 2 == 1:
            x -= 1
    
    ans += (x // 2) * B[i]

print(ans)
#print(B)
#print(A)