using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            new ABC124().SolveC(new ConsoleInput(Console.In, ' '));
        }
    }

    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); } }
        public int ReadInt { get { return int.Parse(Read); } }
        public long ReadLong { get { return long.Parse(Read); } }
        public double ReadDouble { get { return double.Parse(Read); } }
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }

    public static class AtCoderLib
    {
        // ユークリッドの互除法 
        public static int Gcd(int a, int b)
        {
            if (a < b)
                // 引数を入替えて自分を呼び出す
                return Gcd(b, a);
            while (b != 0)
            {
                var remainder = a % b;
                a = b;
                b = remainder;
            }
            return a;
        }
    }
  class ABC124
    {
        public void SolveC(ConsoleInput cin)
        {
            var s = cin.Read;
            var C = s.ToCharArray();
            //0から開始の書き換え数
            int count0 = 0;
            if (C[0] != '0')
            {
                C[0] = '0';
                count0++;
            }
            for (int i = 1; i < C.Length; i++)
            {
                if (C[i] == C[i - 1])
                {
                    if (C[i - 1] == '0') C[i] = '1';
                    else C[i] = '0';
                    count0++;
                }
            }
            C = s.ToCharArray();
            //1から開始の書き換え数
            int count1 = 0;
            if (C[0] != '1')
            {
                C[0] = '1';
                count1++;
            }
            for (int i = 1; i < C.Length; i++)
            {
                if (C[i] == C[i - 1])
                {
                    if (C[i - 1] == '0') C[i] = '1';
                    else C[i] = '0';
                    count1++;
                }
            }
            WriteLine(Min(count0, count1));
            ReadLine();
        }
    }
}


Measured memory_bytes: 13348