using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string ReadLine()
    { 
        return Console.ReadLine().Trim();
    }

    static void Main()
    {
        char[] S = ReadLine().ToArray();
        List<int> idxs = new List<int>();
        for (int i = 0; i < S.Length; i++)
        {
            if (S[i] == '0')
                idxs.Add(i);
        }

        long ans = 0;
        for (int i = 0; i < idxs.Count; i++)
        {
            if (i == 0)
            {
                int v = idxs[0] / 2;
                idxs[0] -= 2 * v;
                ans += v;
            }
            else
            {
                int v  = (idxs[i] - idxs[i-1] - 1) / 2;
                idxs[i] -= 2 * v;
                ans += v;
            }
        }
        Console.WriteLine(ans);
    }
}