#include <iostream>
#include <iomanip>
#include <vector>
#include <valarray>
#include <map>
#include <set>
#include <list>
#include <queue>
#include <stack>
#include <bitset>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <complex>
#include <string>
#include <sstream>

#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cassert>

#include <unordered_set>
#include <unordered_map>
#include <random>
#include <thread>
#include <chrono>

using namespace std;

#define val const auto
#define var auto

#define all(c) c.begin(), c.end()
#define repeat(i, n) for (int i = 0; i < static_cast<int>(n); i++)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define dump(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define dump(x) 
#endif

typedef complex<double> point;

template<typename T,std::size_t N>
struct _v_traits {using type = std::vector<typename _v_traits<T,N-1>::type>;};
template<typename T>
struct _v_traits<T,1> {using type = std::vector<T>;};
template<typename T,std::size_t N=1>
using vec = typename _v_traits<T,N>::type;

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "{";
    for(auto it = v.begin(); it != v.end(); ++it){
        if(it != v.begin()){
            os << ",";
        }
        os << *it;
    }
    return os << "}";
}

template <typename T>
T read_next(){
    T ret; cin >> ret;
    return ret;
}


template<typename T>
void isort(std::vector<T>& v, std::function<bool(T,T)> comp=less<T>()){
    sort(v.begin(), v.end(), comp);
}
template<typename T>
std::vector<T> sort(std::vector<T> v, std::function<bool(T,T)> comp=less<T>()){
    isort(v);
    return v;
}
template<typename T1, typename T2>
std::vector<T2> rmap(const std::vector<T1>& v, std::function<T2(T1)> f){
    std::vector<T2> t; t.reserve(v.size());
    for(const auto& i: v) t.push_back(f(i));
    return t;
}
std::vector<std::string> split(std::string str, char delim){
    std::vector<std::string> res;
    size_t current = 0, found;
    while((found = str.find_first_of(delim, current)) != std::string::npos){
        res.push_back(std::string(str, current, found - current));
        current = found + 1;
    }
    res.push_back(std::string(str, current, str.size() - current));
    return res;
}
string join(const std::vector<string>& v, string delim){
    string ret = "";
    for(auto it = v.begin(); it != v.end(); ++it){
        if(it != v.begin()){
            ret += delim;
        }
        ret += *it;
    }
    return ret;
}


using i64 = int64_t;
using i32 = int32_t;

const int INF = 1000000000;
int calc_two(const string& a,
             const string& b,
             const int H){
    vector<vector<int> > cost(H, vector<int>(H, INF));
    std::function<int(int, int)> calc_cost = [&](int x, int y){
        if(x == H) return 0;
        if(y == H) return 0;
        if(cost[x][y] != INF) return cost[x][y];
        int diff = (a[x] == b[y]);
        int ret = diff + calc_cost(x+1, y+1);
        return cost[x][y] = ret;
    };

    vector<vector<int> > last(H, vector<int>(H, INF));
    std::function<int(int, int)> calc_last = [&](int y, int x){
        if(x == H) return 0;
        if(y == H) return 0;
        if(last[y][x] != INF) return last[y][x];
        int use = calc_cost(y, x);
        int left  = use + calc_last(y+1,  x);
        int right = use + calc_last(y,  x+1);
        return last[y][x] = min(left, right);
    };
    return calc_last(0, 0);
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int H, W; cin >> H >> W;
    vector<string> ci(H);
    for(string& s: ci) cin >> s;
    vector<string> c(W, string(H, '*'));
    for(int y=0;y<H;y++){
        for(int x=0;x<W;x++){
            c[x][y] = ci[y][x];
        }
    }
    for(string &i : c){
        reverse(all(i));
    }
    int ret = 0;
    for(int i=0;i<W-1;i++){
        ret += calc_two(c[i], c[i+1], H);
    }
    cout << ret << endl;
    return 0;
}


Measured memory_bytes: 1424