#include<iostream>

#include<stdio.h>

#define MAXN 20

using namespace std;

int DFS(int i,int j,int len,int r,int c);

bool key[];

char a[MAXN][MAXN];

//bool bool_lean[MAXN][MAXN];

int main()

{

    //freopen("acm.acm","r",stdin);

    int i;

    int j;

    int r;

    int c;

    cin>>r>>c;

//    memset(bool_lean,false,sizeof(bool_lean));

    for(i = ; i < r; ++ i)

    {

        for(j = ; j < c; ++ j)

        {

            cin>>a[i][j];

        }

    }

    key[a[][]-'A'] = true;

    cout<<DFS(,,,r,c)<<endl;

}

int DFS(int i,int j,int len,int r,int c)

{

    int tem;

    tem = len;

    int max = ;

    if(i > )

    {

        if(!key[a[i-][j] - 'A'])

        {

            key[a[i-][j] - 'A'] = true;

        //    bool_lean[i-1][j] = true;

            len = DFS(i-,j,len+,r,c);

            key[a[i-][j] - 'A'] = false;

        //    bool_lean[i-1][j] = false;

            if(len > max)

                max = len;

            len = tem;

        }

    }

    if(i < r-)

    {

        if(!key[a[i+][j] - 'A'])

        {

            key[a[i+][j] - 'A'] = true;

        //    bool_lean[i+1][j] = true;

            len = DFS(i+,j,len+,r,c);

            key[a[i+][j] - 'A'] = false;

        //    bool_lean[i+1][j] = false;

            if(len > max)

                max = len;

            len = tem;

        }

    }

    if(j > )

    {

        if(!key[a[i][j-] - 'A'])

        {

            key[a[i][j-] - 'A'] = true;

        //    bool_lean[i][j-1] = true;

            len = DFS(i,j-,len+,r,c);

            key[a[i][j-] - 'A'] = false;

        //    bool_lean[i][j-1] = false;

            if(len>max)

                max = len;

            len = tem;

        }

    }

    if(j < c-)

    {

        if(!key[a[i][j+] - 'A'])

        {

            key[a[i][j+] - 'A'] = true;

    //        bool_lean[i][j+1] = true;

            len = DFS(i,j+,len+,r,c);

            key[a[i][j+] - 'A'] = false;

    //        bool_lean[i][j+1] = false;

            if(len>max)

                max = len;

            len = tem;

        }

    }

    if(len > max)

        max = len;

    return max;

}