#include <fstream>
using namespace std;

int l, lmax, verif_trecere[26];

void rezolvare (int x, int y, char s[21][21], int R, int C)
{
    if (verif_trecere[s[x][y] - 'A'] || x < 0 || y < 0 || x >= R || y >= C)
        return;
    verif_trecere[s[x][y] - 'A'] = 1;
    ++l;
    if (l > lmax)
        lmax = 1;
    rezolvare(x-1, y, s, R, C);
    rezolvare(x+1, y, s, R, C);
    rezolvare(x, y-1, s, R, C);
    rezolvare(x, y+1, s, R, C);
    verif_trecere[s[x][y] - 'A'] = 0;
    --l;
}

int main()
{
    int R, C;
    char s[21][21];
    ifstream fin("monkey.in");
    ofstream fout("monkey.out");
    fin >> R >> C;
    for (int i = 0; i < R; ++i)
        for (int j = 0; j < C; ++j)
            fin >> s[i][j];
    rezolvare(0, 0, s, R, C);
    fout << lmax;
    fin.close();
    fout.close();
    return 0;
}