#include <iostream>
#include <iomanip>
#include <algorithm>
#include <bitset>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <cmath>
#include <time.h>
#include <random>
#include <string>
#include <cassert>
#include <vector>
#include <ostream>
#include <istream>
#include <stack>
#include <deque>
#include <queue>
#include <functional>
#include <chrono>
#include <stack>

using namespace std;

#define int long long
#define pb push_back
#define all(a) (a).begin(), (a).end()
#define pii pair<int, int>
#define ld long double

ostream& operator<< (ostream &out, const vector<int> &b) {
    for (auto k : b) out << k << " ";
    return out;
}

istream& operator>> (istream& in, pii& b) {
    in >> b.first >> b.second;
    return in;
}

ostream& operator<< (ostream& out, const pii& b) {
    out << "{" << b.first << ", " << b.second << "}";
    return out;
}

template <typename T1, typename T2> inline bool chkmin(T1 &x, const T2 &y) {if (x > y) {x = y; return 1;} return 0;}
template <typename T1, typename T2> inline bool chkmax(T1 &x, const T2 &y) {if (x < y) {x = y; return 1;} return 0;}


#ifdef LOCAL
    #define dbg(x) cout << #x << " : " << (x) << "\n";
    const int INF = 1e18;
    // const int mod = 2600000069;
    // const int p = 10;
#else
    #define dbg(x) 57
    const int INF = 1e18;
    // const int mod = 2600000069;
    // const int p = 179;
#endif

const ld PI = acos(-1);

#define time clock() / (double) CLOCKS_PER_SEC

// #pragma GCC optimize("Ofast,no-stack-protector")
// #pragma GCC target("sse,sse2,sse3,sse3,sse4")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC optimize("fast-math")
// #pragma GCC target("avx2")  
// #pragma GCC optimize("section-anchors")
// #pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
// #pragma GCC optimize("vpt")
// #pragma GCC optimize("rename-registers")
// #pragma GCC optimize("move-loop-invariants")
// #pragma GCC optimize("unswitch-loops")
// #pragma GCC optimize("function-sections")
// #pragma GCC optimize("data-sections")

mt19937 gen(chrono::high_resolution_clock::now().time_since_epoch().count());

const int MAXN = 310;

int n, m;
int a[MAXN][MAXN];
int cost[MAXN][MAXN];
int dp[MAXN][MAXN];

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> n >> m;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            char c;
            cin >> c;
            a[i][j] = c - 'a';
        }
    }
    int ans = 0;
    for (int i = 0; i < m - 1; i++) {
        for (int j = 0; j <= n; j++) {
            for (int k = 0; k <= n; k++) {
                dp[j][k] = INF;
                cost[j][k] = 0;
            }
        }
        for (int h1 = 1; h1 <= n; h1++) {
            for (int h2 = 1; h2 <= n; h2++) {
                cost[h1][h2] = cost[h1 - 1][h2 - 1] + (a[h1 - 1][i] == a[h2 - 1][i + 1]);
            }
        }
        dp[n][n] = 0;
        for (int i = n; i >= 0; i--) {
            for (int j = n; j >= 0; j--) {
                if (i > 0) {
                    chkmin(dp[i - 1][j], dp[i][j] + cost[i][j]);
                }
                if (j > 0) {
                    chkmin(dp[i][j - 1], dp[i][j] + cost[i][j]);
                }
            }
        }
        ans += dp[0][0];
    }
    cout << ans << "\n";
}
/*

*/


Measured memory_bytes: 5772