#include<iostream>

#include<map>

using namespace std;

#define ll long long

#define ull unsigned long long

const int maxn = ;

const ull base1 = ;

const ull base2 = ;

int n, m;

char mp[maxn][maxn];

ull has[maxn][maxn];

ull p1[maxn], p2[maxn];

map<ull, int>mmp;

void init(){

    p1[] = p2[] = ;

    for (int i = ; i <= ; ++i){

        p1[i] = p1[i - ] * base1;

        p2[i] = p2[i - ] * base2;

    }

}

void Hash(){

    has[][] = ;

    has[][] = ;

    has[][] = ;

    for (int i = ; i <= n;++i)

    for (int j = ; j <= m; ++j){

        has[i][j] = has[i][j-] * base1 + mp[i][j]-'a';

    }

    for (int i = ; i <= n;++i)

    for (int j = ; j <= m; ++j){

        has[i][j] = has[i - ][j] * base2 + has[i][j];

    }

}

bool check(int x){

    mmp.clear();

    for (int i = x; i <= n; ++i)

    for (int j = x; j <= m; ++j){

        ull k = has[i][j] - has[i - x][j] * p2[x] - has[i][j - x] * p1[x] + has[i - x][j-x] * p1[x]*p2[x];

        mmp[k]++;

        if (mmp[k] >= )return true;

    }

    return false;

}

int main()

{

    init();

    while (cin >> n >> m){

        for (int i = ; i <= n; ++i)

            cin >> (mp[i] + );

        Hash();

        int l = , r = , ans = ;

        while (l <= r){

            int mid = (l + r) / ;

            if (check(mid)){

                l = mid + ;

                ans = mid;

            }

            else{

                r = mid - ;

            }

        }

        cout << ans << endl;

    }

}