#pragma GCC optimize("Ofast")

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <random>

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;

typedef
tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> orderedSet;

#define pb push_back
#define F first
#define S second
#define all(a) (a).begin(), (a).end()

#define for1(i0, l0, r0) for (int i0 = l0; i0 < r0; ++i0)
#define for2(i0, l0, r0) for (int i0 = l0; i0 <= r0; ++i0)
#define forn(i0, n0) for (int i0 = 0; i0 < n0; ++i0)
#define forn1(i0, n0) for (int i0 = 1; i0 < n0; ++i0)
#define forr(i0, n0) for (int i0 = n0; i0 >= 0; --i0)
#define forr1(i0, r0, l0) for (int i0 = r0; i0 > l0; --i0)
#define forr2(i0, r0, l0) for (int i0 = r0; i0 >= l0; --i0)

#define Sort(a) sort(all(a))
#define Reverse(a) reverse(all(a))

typedef long double ld;
#define ui unsigned int
#define ull unsigned long long

typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
typedef pair<string, string> pss;

typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<double> vd;
typedef vector<ld> vld;
typedef vector<bool> vb;
typedef vector<string> vs;

typedef vector<pii> vpii;
typedef vector<pll> vpll;
typedef vector<pdd> vpdd;

typedef vector<vi> vvi;
typedef vector<vd> vvd;
typedef vector<vll> vvll;
typedef vector<vb> vvb;

#define vpss vector<pss>
#define vvs vector<vs>
#define vvpii vector<vpii>
#define vvpll vector<vpll>
#define vpt vector<pt>
#define vvvi vector<vvi>
#define vsi vector<set<int>>
#define ss second
#define ff first

#define printvi(arr) for (int x0 : arr) cout << x0 << ' '; cout << '\n';
#define printvll(arr) for (ll x0 : arr) cout << x0 << ' '; cout << '\n';
#define printpair(pair0) cout << pair0.F << ' ' << pair0.S << '\n';
#define printvp(arr) for (auto pair0 : arr) printpair(pair0);
#define initArray(arr, N0, X0) memset(arr, X0, N0 * sizeof(int))
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

void init() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    //cout.precision(16);
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
}

const int maxn = 303;

int f(string s1, string s2) {
    int n = s1.length();
    vvi dp(n + 1, vi(n + 1));
    dp[n][n] = 0;
    vpii xy;
    vi T(2 * maxn);
    vvi sum(n + 1, vi(n + 1));
    forn(i, n) {
        forn(j, n) {
            if (s1[i] == s2[j]) {
                T[i - j + maxn]++;
                sum[i][j]++;
            }
        }
    }
    forr(i, n) {
        forr(j, n) {
            if (i == n && j == n) continue;
            int x1 = 1e9;
            int x2 = 1e9;
            if (i < n) {
                x1 = dp[i + 1][j] + T[i + 1 - j + maxn];
            }
            if (j < n) {
                x2 = dp[i][j + 1] + T[i - j - 1 + maxn];
            }
            T[i - j + maxn] -= sum[i][j];
            dp[i][j] = min(x1, x2);
        }
    }
    return dp[0][0];
}

void solve() {
    int n, m;
    cin >> n >> m;
    vs a(n), a1(m);
    forn(i, n) cin >> a[i];
    forn(i, n) {
        forn(j, m) a1[j] += a[i][j];
    }
    int ans = 0;
    forn(i, m - 1){
        ans += f(a1[i], a1[i + 1]);
    }
    cout << ans << '\n';
}

bool multitest = false;

int main() {
    init();
    int t = 1;
    if (multitest) cin >> t;
    forn(i, t) solve();
}

Measured memory_bytes: 4244