s = gets.chomp+"00"
n = s.size
ans = 0
cnt = 0
a = []
n.times do |i|
    if s[i] == "1"
        cnt += 1
    elsif i > 0
        if s[i-1] == "1"
            a << (cnt < 2 ? cnt : 2)
        elsif !a.empty?
            b = a.inject(:+)
            ans += (b < a.size+1 ? b : a.size+1)
            a = []
        end
        cnt = 0
    end
end
puts ans