local s = io.read()
local n = #s
local t = {}
local curlen = 1
for i = 2, n do
  if s:sub(i, i) == "0" then
    table.insert(t, curlen)
    curlen = 0
  else
    if curlen == 0 then table.insert(t, 0) end
    curlen = curlen + 1
  end
end
table.insert(t, curlen)
table.insert(t, 0)
local cnt = 0
local curleft = 1
local curpos = 2
while curpos <= #t do
  if curpos == #t or t[curpos + 1] == 0 then
    if 1 < t[curleft] then
      cnt = cnt + 2
    else
      cnt = cnt + 1
    end
    while curpos <= #t and t[curpos] == 0 do
      curpos = curpos + 1
    end
    curleft = curpos
    curpos = curleft + 1
  else
    if 1 < t[curleft] or 1 < t[curpos] then
      cnt = cnt + 1
      t[curleft] = 2
      curpos = curpos + 2
    else
      cnt = cnt + 1
      curleft = curpos
      curpos = curpos + 2
    end
  end
end
print(cnt)