using System;
using System.Collections.Generic;
using System.IO;
using System.Runtime.Remoting.Contexts;
using Microsoft.Win32;

namespace Contest
{
    class Program
    {
        private Scanner sc;

        public void Solve()
        {
            sc = new Scanner();
            string S = sc.Next();

            int a = 0;
            int b = 0;
            for (int i = 0; i < S.Length; i++)
            {
                if (i % 2 == S[i] - '0')
                {
                    a++;
                }
                else
                {
                    b++;
                }
            }
            Console.WriteLine(Math.Min(a,b));
        }

        static void Main() => new Program().Solve();
    }
}

struct ModInt
{
    public long Num;
    public const long Mod = (long)1e9 + 7;
    public ModInt(long n) { Num = n % Mod; }

    public override string ToString() { return Num.ToString(); }
    public override bool Equals(object b) { return Num.Equals(b); }

    public static ModInt operator +(ModInt a, ModInt b)
    {
        return (a.Num + b.Num) % Mod;
    }
    public static ModInt operator -(ModInt a, ModInt b)
    {
        return (a.Num - b.Num) % Mod;
    }
    public static ModInt operator *(ModInt a, ModInt b)
    {
        return (a.Num * b.Num) % Mod;
    }
    public static ModInt operator %(ModInt a, long b)
    {
        return ((a.Num % b) + Mod) % Mod;
    }
    public static ModInt operator /(ModInt a, ModInt b)
    {
        return a * ModInt.Pow(b, Mod - 2);
    }

    public static ModInt Pow(ModInt a, long b)
    {
        if (b == 0) return 1;
        return (b % 2 == 1 ? a : 1) * Pow(a * a, b / 2);
    }

    public static implicit operator ModInt(long n) { return new ModInt(n); }


}

class Scanner
{
    public Scanner()
    {
        _stream = new StreamReader(Console.OpenStandardInput());
        _pos = 0;
        _line = new string[0];
        _separator = ' ';
    }
    private readonly char _separator;
    private readonly StreamReader _stream;
    private int _pos;
    private string[] _line;
    #region get a element
    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = _stream.ReadLine().Split(_separator);
            _pos = 0;
        }
        return _line[_pos++];
    }
    public int NextInt()
    {
        return int.Parse(Next());
    }
    public long NextLong()
    {
        return long.Parse(Next());
    }
    public double NextDouble()
    {
        return double.Parse(Next());
    }
    #endregion
    #region convert array
    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }
    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }
    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }
    #endregion
    #region get array
    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = _stream.ReadLine().Split(_separator);
        _pos = _line.Length;
        return _line;
    }
    public int[] IntArray()
    {
        return ToIntArray(Array());
    }
    public long[] LongArray()
    {
        return ToLongArray(Array());
    }
    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }
    #endregion
}

Measured memory_bytes: 13108