using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;

namespace AtCoderProject
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine(new Program().Calc());
        }

        public object Calc()
        {
            var tiles = ReadLine().Select(c => c == '1').ToArray();

            int 偶数番目の黒か奇数番目の白 = 0;

            if (tiles.Length % 2 == 1)
            {
                if (!tiles[tiles.Length - 1])
                    偶数番目の黒か奇数番目の白++;
            }

            for (int i = 0; i < tiles.Length - 1; i += 2)
            {
                if (!tiles[i])
                    偶数番目の黒か奇数番目の白++;
                if (tiles[i + 1])
                    偶数番目の黒か奇数番目の白++;
            }
            return Math.Min(偶数番目の黒か奇数番目の白, tiles.Length - 偶数番目の黒か奇数番目の白);
        }

#if DEBUG
#pragma warning disable IDE0051 // 使用されていないプライベート メンバーを削除する
#pragma warning disable IDE1006 // 命名スタイル
        static string _ReadLineImpl() => queue.Dequeue();
#pragma warning restore IDE1006 // 命名スタイル
        static readonly Queue<string> queue;
        static Program()
        {
            string input = @"
00010
";
            queue = new Queue<string>();
            foreach (var line in input.Split(new char[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries))
                queue.Enqueue(line);
        }
#else
        private static string _ReadLineImpl() => Console.ReadLine();
#endif
        static string ReadLine() => _ReadLineImpl();
        static int ReadLineInt32() => int.Parse(ReadLine());
        static long ReadLineInt64() => long.Parse(ReadLine());
        static ulong ReadLineUInt64() => ulong.Parse(ReadLine());
        static double ReadLineDouble() => double.Parse(ReadLine());
        static List<string> ReadLineSplit() => ReadLine().Split().ToList();
        static List<int> ReadLineSplitInt32() => ReadLine().Split().Select(s => int.Parse(s)).ToList();
        static List<long> ReadLineSplitInt64() => ReadLine().Split().Select(s => long.Parse(s)).ToList();
        static List<ulong> ReadLineSplitUInt64() => ReadLine().Split().Select(s => ulong.Parse(s)).ToList();
        static List<double> ReadLineSplitDouble() => ReadLine().Split().Select(s => double.Parse(s)).ToList();

        static IEnumerable<string> RepeatReadLines(int count)
        {
            for (int i = 0; i < count; i++)
                yield return ReadLine();
        }
        static IEnumerable<int> RepeatReadLinesInt32(int count)
        {
            for (int i = 0; i < count; i++)
                yield return ReadLineInt32();
        }
        static IEnumerable<List<string>> RepeatReadLinesSplit(int count)
        {
            for (int i = 0; i < count; i++)
                yield return ReadLineSplit();
        }
    }
    static class BitArrayUtils
    {
        static BitArray ToBitArray(this long l)
        {
            return new BitArray(BitConverter.GetBytes(l));
        }
        static long ToInt64(this BitArray bitArray)
        {
            var array = new byte[8];
            bitArray.CopyTo(array, 0);
            return BitConverter.ToInt64(array, 0);
        }
        static BitArray ToBitArray(this ulong l)
        {
            return new BitArray(BitConverter.GetBytes(l));
        }
        static ulong ToUInt64(this BitArray bitArray)
        {
            var array = new byte[8];
            bitArray.CopyTo(array, 0);
            return BitConverter.ToUInt64(array, 0);
        }
    }
}


Measured memory_bytes: 13388