#include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
#include <map>
#include <algorithm>
#include <string>
#include <utility>
#include <set>
#include <stack>
#include <deque>
#include <ctime>
#include <random>
#include <cassert>
#include <cmath>
#include <climits>
#include <queue>
#include <cstring>
#include <bitset>
#include <iomanip>
#include <chrono>

/*
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("popcnt")
#pragma GCC target("avx2")
*/


#ifdef LOCAL
    #define dbg(x) cout << #x << " : " << x << endl;
#else 
    #define dbg(x)
#endif

#define pb push_back
#define ppb pop_back()
#define mp make_pair
#define fi(a,b) for (int i=a;i<b;i++)
#define fj(a,b) for (int j=a;j<b;j++)
#define fk(a,b) for (int k=a;k<b;k++)
#define fi1(a,b) for (int i=a-1;i>=b;i--)
#define fj1(a,b) for (int j=a-1;j>=b;j--)
#define fk1(a,b) for (int k=a-1;k>=b;k--)
#define fx(x,a) for (auto& x : a)
#define rep(i, a, b) for (int i = a; i < b; ++i)
#define rep1(i, a, b) for (int i = a - 1; i >= b; --i)
#define siz(x) (int)x.size()
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()

using namespace std;

template<typename T1, typename T2>inline void mine(T1 &x, T2 y) { if (x > y) x = y; }
template<typename T1, typename T2>inline void maxe(T1 &x, T2 y) { if (x < y) x = y; }

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


typedef long long ll;
typedef unsigned long long ull;
typedef char ch;
typedef string str;
typedef pair<int, int> pii;
typedef map<int, int> mii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<pii> vpii;
typedef vector<vpii> vvpii;
typedef vector<ch> vch;
typedef vector<vch> vvch;
typedef vector<str> vs;



const int MOD = 1000000007;
const int INF = 1000000050;
const long long BIG = (long long)2e18 + 50;
const int MX = 310;
const double EPS = 1e-9;


mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());


str s[MX];
int dp[MX][MX];
int kek[2 * MX][MX];

int32_t main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
     
    int len, n;
    cin >> len >> n;
    fi(0, n)
        s[i] = str(len, '0');
    fi(0, len) {
        fj(0, n) {
            cin >> s[j][i];
        }
    }
    int ans = 0;
    fi(1, n) {
        dbg(i);
        memset(kek, 0, sizeof(kek));
        fj(0, len) {
            fk(0, len) {
                kek[j - k + MX][j] = s[i - 1][j] == s[i][k];
            }
        }
        fj(0,  2 * MX)
            partial_sum(kek[j], kek[j] + MX, kek[j]);
        fj(0, MX) fk(0, MX) dp[j][k] = INF;
        dp[len][len] = 0;
        fj1(len + 1, 0) {
            dbg(j);
            fk1(len + 1, 0) {
                dbg(k);
                dbg(dp[j][k]);
                int w = dp[j][k];
                if (j) {
                    dbg(kek[j - k + MX][j - 1]);
                    w += kek[j - k + MX][j - 1];
                    mine(dp[j - 1][k], w);
                }

                dbg(w);
                if (k)
                    mine(dp[j][k - 1], w);
            }
        }

        ans += dp[0][0];
        dbg(ans);
    }
    cout << ans << '\n';
}

Measured memory_bytes: 4852