#include <bits/stdc++.h>
#include <unistd.h>
#include <sys/time.h>
#include <stdlib.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<P, ll> T;
typedef pair<long double , ll> Ps;
typedef pair<ll, bool> Pb;

const ll INF = 1e18;
const ll fact_table = 3200008;
long double Pi = 3.1415926535897932384626;

priority_queue <ll> pql;
priority_queue <P> pqp;
priority_queue <P> bag;
//big priority queue
priority_queue <ll, vector<ll>, greater<ll> > pqls;
priority_queue <P, vector<P>, greater<P> > pqps;
//small priority queue
//top pop

ll dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
ll dy[8] = {0, 1, 0, -1, 1, -1, -1, 1};
//↓,→,↑,←

/*
#define endl "\n"
#ifdef ENJAPMA
#undef endl
#endif
*/
#define p(x) cout<<x<<endl;
#define el cout<<endl;
#define pe(x) cout<<x<<" ";
#define ps(x) cout<<fixed<<setprecision(25)<<x<<endl;
#define pu(x) cout<<(x);
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define CLEAR(a) a = decltype(a)();
#define pc(x) cout << x << ",";
#define rep(i, n) for (ll i = 0; i < (n); i ++)

// const ll mod = 998244353ll;
const ll mod = 1000000007ll;

ll mypow(ll a, ll b, ll mod) {ll x = 1; while (b) {while (!(b & 1)) {(a *= a) %= mod; b >>= 1;}(x *= a) %= mod; b--;} return x;}
void YES(bool cond) { if (cond) { p("YES");} else { p("NO");} return;}
void Yes(bool cond) { if (cond) { p("Yes");} else { p("No");} return;}
void line() { p("--------------------"); return;}

/*
ll fact[fact_table + 5], rfact[fact_table + 5];

void c3_init() {
    fact[0] = rfact[0] = 1;
    for (ll i = 1; i <= fact_table; i++) {
        fact[i] = (fact[i - 1] * i) % mod;
    }
    rfact[fact_table] = mypow(fact[fact_table], mod - 2, mod);
    for (ll i = fact_table; i >= 1; i--) {
        rfact[i - 1] = rfact[i] * i;
        rfact[i - 1] %= mod;
    }
    return;
}
ll c3(ll n, ll r) {
    return (((fact[n] * rfact[r]) % mod ) * rfact[n - r]) % mod;
}
*/
struct Timer {
    int64_t start;
    const int64_t CYCLES_PER_SEC = 2800000000;
    Timer() { reset(); }
    void reset() { start = getCycle(); }
    inline double get() { return (double)(getCycle() - start) / CYCLES_PER_SEC; }
    inline int64_t getCycle() {
        uint32_t low, high;
        __asm__ volatile ("rdtsc" : "=a" (low), "=d" (high));
        return ((int64_t)low) | ((int64_t)high << 32);
    }
};

Timer timer;

bool multicase = false;

ll n, m, k, w, num, a, b, c, d, e, h, q, ans, sum, l;

typedef vector<ll> vec;
typedef vector<vector<ll>> mat;

ll table[505][505];
ll x[505], y[505];
bool edge[705][705];

string s, t;

ll cost_calc(ll d1, ll d2, vec& w1, vec& w2) {
    // ここの計算をO(1)
    ll same = 0;
    for (int i = 0; i < h; i++) {
        ll id1 = i + d1, id2 = i + d2;
        if (id1 >= h || id2 >= h) continue;
        if (w1[h - 1 - id1] == w2[h - 1 - id2]) same ++;
    }
    return same;
}

ll calc(ll w11, ll w12, mat& table) {
    // h1行とh2行を最適に潰した場合の答えを求める
    mat dp = mat(h + 1, vec(h + 1, INF));
    mat cost_table = mat(h + 1, vec(h + 1, 0));

    vec w1 = table[w11], w2 = table[w12];

    // h1 - i, h2 - j こ潰した時の最適値

    for (int i = -h; i <= h; i++) {
        ll cost = 0;
        for (int j = -1; j < h; j++) {
            int id1 = j, id2 = j + i;

            if (0 <= id2 && id2 < h) {
                if (w1[id1] == w2[id2]) cost ++;
            }
            int rd1 = h - id1 - 1;
            int rd2 = h - id2 - 1;
            if (0 <= rd1 && rd1 <= h && 0 <= rd2 && rd2 <= h) {
                cost_table[rd1][rd2] = cost;
            }
        }
    }
    dp[0][0] = 0;
    for (int i = 0; i <= h; i++) {
        for (int j = 0; j <= h; j++) {
            if (i == h && j == h) continue;
            // dp[i][j] -> dp[i+1][j], dp[i][j] -> dp[i][j+1]
            ll cost = cost_table[i][j];
            if (i != h) dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + cost);
            if (j != h) dp[i][j + 1] = min(dp[i][j + 1], dp[i][j] + cost);
        }
    }
    return dp[h][h];

}

void solve() {
    cin >> h >> w;
    mat table = mat(w, vec(h, 0));
    for (int i = 0; i < h; i++) {
        cin >> s;
        for (int j = 0; j < w; j++) {
            table[j][i] = s[j] - 'a' + 1;
        }
    }
    ll ans = 0;
    for (int i = 0; i < w - 1; i++) {
        ans += calc(i, i + 1, table);
    }
    p(ans);
    return;
}

int main() {
    // init();
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll q, testcase = 1;
    if (multicase) {
        cin >> q;
    } else {
        q = 1;
    }
    while (q--) {
        // pu("Case ");pu("#");pu(testcase);pu(": ");
        solve();
        testcase++;
    }
    // solve();
    return 0;
}



Measured memory_bytes: 5296