export {};

const input: string[] = require("fs").readFileSync(0).toString().trim().split('\n');
const [R, C]: number[] = input[0].split(' ').map(Number);

let board: string[][] = [];
for(let i=1; i<=R; i++) {
    board.push(input[i].split(''));
}

let alphabet: boolean[] = new Array(26).fill(false);
alphabet[board[0][0].charCodeAt(0) - 65] = true;
let maxStep: number = 1;

const dx: number[] = [1,-1,0,0];
const dy: number[] = [0,0,1,-1];

const dfs = ([cx, cy, cs]: number[]) => {
    maxStep = Math.max(cs, maxStep);

    for(let i=0; i<4; i++) {
        const nx = cx + dx[i];
        const ny = cy + dy[i];

        if(0<=nx && nx<C && 0<=ny && ny<R && !alphabet[board[ny][nx].charCodeAt(0) - 65]) {
            alphabet[board[ny][nx].charCodeAt(0) - 65] = true;
            dfs([nx,ny,cs+1]);
            alphabet[board[ny][nx].charCodeAt(0) - 65] = false;
        }
    }
}

dfs([0,0,1]);

console.log(maxStep);