import copy

s = input()
s_sub = copy.deepcopy(s)

tar1 = "01"
tar2 = "10"
tar3 = ["1010","1001","0101","0110"]

while((tar1 in s )or(tar2 in s)):
    for i in range(4):
        if tar3[i] in s:
            s = s.replace(tar3[i],"")
            
    if tar1 in s:
        s = s.replace(tar1,"")
    if tar2 in s:
        s = s.replace(tar2,"")



print(len(s_sub)-len(s))
