using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Diagnostics;
using Enu = System.Linq.Enumerable;
using System.Numerics;

public class Program
{
    static readonly int INF = (int)1e9;

    public void Solve()
    {
        int H = Reader.Int();
        int W = Reader.Int();
        var rowBoard = Reader.StringArray(H);
        var cols = Enu.Range(0, W).Select(c => new char[H]).ToArray();
        for (int c = 0; c < W; c++)
            for (int r = 0; r < H; r++)
                cols[c][r] = rowBoard[r][c];

        int ans = 0;
        for (int c = 0; c + 1 < W; c++)
        {
            var A = cols[c];
            var B = cols[c + 1];
            var sameCount = new int[H + 1, H + 1];
            for (int ai = 1; ai <= H; ai++)
                for (int bi = 1; bi <= H; bi++)
                {
                    sameCount[ai, bi] = (A[ai - 1] == B[bi - 1] ? 1 : 0);
                    sameCount[ai, bi] += sameCount[ai - 1, bi - 1];
                }
            var dp = new int[H + 1, H + 1];
            Fill(dp, INF);
            dp[H, H] = 0;

            for (int ai = H; ai >= 0; ai--)
                for (int bi = H; bi >= 0; bi--)
                {
                    int cost = sameCount[ai, bi];
                    if (ai > 0)
                        CheckMin(ref dp[ai - 1, bi], dp[ai, bi] + cost);
                    if (bi > 0)
                        CheckMin(ref dp[ai, bi - 1], dp[ai, bi] + cost);
                }

            ans += dp[0, 0];
        }

        Console.WriteLine(ans);
    }

    bool CheckMin(ref int a, int b) { if (b < a) { a = b; return true; } return false; }

    void Fill<T>(T[,] A, T val)
    {
        for (int a = 0; a < A.GetLength(0); a++)
            for (int b = 0; b < A.GetLength(1); b++)
                A[a, b] = val;
    }
}


class Entry { static void Main() { new Program().Solve(); } }
class Reader
{
    static TextReader reader = Console.In;
    static readonly char[] separator = { ' ' };
    static readonly StringSplitOptions op = StringSplitOptions.RemoveEmptyEntries;
    static string[] A = new string[0];
    static int i;
    static void Init() { A = new string[0]; }
    public static void Set(TextReader r) { reader = r; Init(); }
    public static void Set(string file) { reader = new StreamReader(file); Init(); }
    public static bool HasNext() { return CheckNext(); }
    public static string String() { return Next(); }
    public static int Int() { return int.Parse(Next()); }
    public static long Long() { return long.Parse(Next()); }
    public static double Double() { return double.Parse(Next()); }
    public static int[] IntLine() { return Array.ConvertAll(Split(Line()), int.Parse); }
    public static int[] IntArray(int N) { return Range(N, Int); }
    public static int[][] IntTable(int H) { return Range(H, IntLine); }
    public static string[] StringArray(int N) { return Range(N, Next); }
    public static string[][] StringTable(int N) { return Range(N, () => Split(Line())); }
    public static string Line() { return reader.ReadLine().Trim(); }
    static string[] Split(string s) { return s.Split(separator, op); }
    static T[] Range<T>(int N, Func<T> f) { var r = new T[N]; for (int i = 0; i < N; r[i++] = f()) ; return r; }
    static string Next() { CheckNext(); return A[i++]; }
    static bool CheckNext()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return CheckNext();
        A = Split(line);
        i = 0;
        return true;
    }
}

Measured memory_bytes: 22136