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

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            ABC147.B();
        }

        class ABC147
        {
            public static void A()
            {
                Console.WriteLine(Read.Int() + Read.Int() + Read.Int() >= 22 ? "bust" : "win");
            }
            public static void B()
            {
                var S = Read.Str();
                int count = 0;
                for(int i = 0; i < S.Length / 2; ++i)
                {
                    if (S[i] != S[S.Length - 1 - i]) { count++; }
                }
                Console.WriteLine(count);
            }
            public static void C()
            {
                var N = Read.Int();
                List<Tuple<int, int>[]> XYs = new List<Tuple<int, int>[]>();
                for(int i = 0; i < N; ++i)
                {
                    var A = Read.Int();
                    XYs.Add(Read.TupleInt(A));
                }
                int max = 0;
                for(int i = 0; i < (1 << N); ++i)
                {
                    int count = 0;
                    bool ok = true;
                    for(int j = 0; j < N; ++j)
                    {
                        if ((i & (1 << j)) > 0)
                        {
                            count++;
                            foreach(var XY in XYs[j])
                            {
                                var x = XY.Item1 - 1;
                                var y = XY.Item2;
                                if((i & (1 << x)) > 0 != (y == 1))
                                {
                                    ok = false;
                                }
                            }
                        }
                    }
                    if (ok) { max = Math.Max(max, count); }
                }
                Console.WriteLine(max);
            }
            public static void D()
            {
                var N = Read.Int();
                var A = Read.LongN();
                var count1 = new long[61];
                foreach(var a in A)
                {
                    for(int i = 0; i < count1.Length; ++i)
                    {
                        if((a & (1L << i)) > 0) { count1[i]++; }
                    }
                }
                long sum = 0;
                long pow2 = 1;
                for(int i = 0; i < count1.Length; ++i)
                {
                    sum += (((count1[i] * (N - count1[i])) % Util.P) * pow2) % Util.P;
                    pow2 = (pow2 * 2) % Util.P;
                }
                Console.WriteLine(sum % Util.P);
            }
            public static void E()
            {
                var H = Read.Int();
                var W = Read.Int();
                var A = Read.LongMatrix(H, W);
                var B = Read.LongMatrix(H, W);
                HashSet<long>[,] dp = new HashSet<long>[H, W];
                for(int i = 0; i < H; ++i)
                {
                    for(int j = 0; j < W; ++j)
                    {
                        var s = new HashSet<long>();
                        dp[i, j] = s;
                        var diff = A[i, j] - B[i, j];
                        if (i == 0 && j == 0) { s.Add(diff); if (diff != 0) { s.Add(-diff); } }
                        else
                        {
                            if (i > 0)
                            {
                                var prev = dp[i - 1, j];
                                foreach (var p in prev)
                                {
                                    var v1 = p + diff; if (!s.Contains(v1)) { s.Add(v1); }
                                    var v2 = p - diff; if (!s.Contains(v2)) { s.Add(v2); }
                                }
                            }
                            if (j > 0)
                            {
                                var prev = dp[i, j - 1];
                                foreach (var p in prev)
                                {
                                    var v1 = p + diff; if (!s.Contains(v1)) { s.Add(v1); }
                                    var v2 = p - diff; if (!s.Contains(v2)) { s.Add(v2); }
                                }
                            }
                        }
                    }
                }
                Console.WriteLine(dp[H - 1, W - 1].Select(x => Math.Abs(x)).Min());
            }
            public static void F()
            {

            }
        }
        class Util
        {
            public static readonly long P = 1000000007;

            public static long GCD(long a, long b)
            {
                while (b > 0)
                {
                    var c = a % b;
                    a = b;
                    b = c;
                }
                return a;
            }

        }
        class Graph
        {
        }

        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;
            }
            public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
            {
                Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new Dictionary<int, long>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1], ab[2]);
                    dict[ab[1]].Add(ab[0], ab[2]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 13268