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
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var N = cin.ReadLine;
            var charArray = N.ToCharArray();
            var charArray2 = N.ToCharArray();
            long counter = charArray.Count();
            long result1 = 0;
            long result2 = 0;
            if (charArray2[0] == '0')
            {
                charArray2[0] = '1';
                ++result2;
            }
            else
            {
                charArray2[0] = '0';
                ++result2;
            }

            for (int i = 0; i < counter-1; i++)
            {
                if (charArray[i] == charArray[i+1])
                {
                    if (charArray[i+1] == '0')
                    {
                        charArray[i + 1] = '1';
                    }
                    else
                    {
                        charArray[i + 1] = '0';
                    }
                    ++result1;
                }

                if (charArray2[i] == charArray2[i + 1])
                {
                    if (charArray2[i + 1] == '0')
                    {
                        charArray2[i + 1] = '1';
                    }
                    else
                    {
                        charArray2[i + 1] = '0';
                    }
                    ++result2;
                }
            }
            if (result1 > result2)
            {
                Console.Write(result2.ToString());
            }
            else
            {
                Console.Write(result1.ToString());
            }
        }
    }
    public class deci
    {
        public bool IsFinish { get; set; } = false;
        public int Len { get; set; }
    }
    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; }
    }
}

Measured memory_bytes: 11872