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

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            ABC124.C();
        }

        class ABC124
        {
            public static void A()
            {
                var A = Read.Int();
                var B = Read.Int();
                Console.WriteLine(Math.Max(Math.Max(A, B) * 2 - 1, A + B));
            }
            public static void B()
            {
                var N = Read.Int();
                var H = Read.IntN();
                int max = 0;
                int count = 0;
                for(int i = 0; i < N; ++i)
                {
                    if (max <= H[i]) { count++; }
                    max = Math.Max(max, H[i]);
                }
                Console.WriteLine(count);
            }
            public static void C()
            {
                var S = Read.Str();
                int count0 = 0, count1 = 0;
                for(int i = 0; i < S.Length; ++i)
                {
                    if (i % 2 == 0 ^ S[i] == '0')
                    {
                        count0++;
                    }
                    else
                    {
                        count1++;
                    }
                }
                Console.WriteLine(Math.Min(count0, count1));
            }
            public static void D()
            {
                var N = Read.Int();
                var K = Read.Int();
                var S = Read.Str();
                List<int> stand = new List<int>();
                List<int> reverse = new List<int>();
                int s = 0, r = 0;
                bool prevR = true;
                foreach(var c in S)
                {
                    if (prevR)
                    {
                        if (c == '1')
                        {
                            r++;
                        }
                        else
                        {
                            reverse.Add(r);
                            r = 0; s = 1;
                            prevR = false;
                        }
                    }
                    else
                    {
                        if (c == '0')
                        {
                            s++;
                        }
                        else
                        {
                            stand.Add(s);
                            s = 0;r = 1;
                            prevR = true;
                        }
                    }
                }
                if (prevR) { reverse.Add(r); } else { stand.Add(s);reverse.Add(0); }
                if (stand.Count <= K)
                {
                    Console.WriteLine(N);return;
                }
                int count = stand.Take(K).Sum() + reverse.Take(K + 1).Sum();
                int max = count;
                for(int i = K; i < stand.Count; ++i)
                {
                    count += reverse[i + 1] + stand[i];
                    count -= reverse[i - K] + stand[i - K];
                    max = Math.Max(max, count);
                }
                Console.WriteLine(max);
            }
        }

        class Util
        {
        }

        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new HashSet<int>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1]);
                    dict[ab[1]].Add(ab[0]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 11744