//
//  main.cpp
//
//  ******************************
//  Рекурсия
//  5. Самый длинный связный кусок
//  ******************************
//
//  Created by Mikhail Mustakimov on 09.03.2018.
//  Copyright © 2018 Mikhail Mustakimov. All rights reserved.
//

#include <iostream>
#include <fstream>

using namespace std;

void readMatrix(ifstream& ifs, short *arr, size_t m, size_t n) {
    for (size_t i = 0; i < m; i++) {
        for (size_t j = 0; j < n; j++)
            ifs >> arr[i * m + j];
    }
}

void printMatrix(short *arr, size_t m, size_t n) {
    for (size_t i = 0; i < m; i++) {
        for (size_t j = 0; j < n; j++)
            cout << arr[i * m + j] << " ";
        cout << endl;
    }
}

unsigned count(size_t y, size_t x, short *arr, size_t m, size_t n) {
    int res = 0;

    if (arr[y * m + x] == 0)
        return 0;

    if (x < (n - 1) && arr[y * m + (x + 1)]) {
        arr[y * m + x] = 0;
        res += count(y, x + 1, arr, m, n);
    }
    if (x > 0 && arr[y * m + (x - 1)]) {
        arr[y * m + x] = 0;
        res += count(y, x - 1, arr, m, n);
    }
    if (y < (m - 1) && arr[(y + 1) * m + x]) {
        arr[y * m + x] = 0;
        res += count(y + 1, x, arr, m, n);
    }
    if (y > 0 && arr[(y - 1) * m + x]) {
        arr[y * m + x] = 0;
        res += count(y - 1, x, arr, m, n);
    }
    arr[y * m + x] = 0;
    return res + 1;
}

unsigned getLongestWay(short *arr, size_t m, size_t n) {
    unsigned res = 0;

    for (size_t i = 0; i < m; i++) {
        for (size_t j = 0; j < n; j++) {
            res = max(res, count(i, j, arr, m, n));
//            cout << "i=" << i << "; j=" << j << "; res=" << res << endl;
//            printMatrix(arr, m, n);
//            cout << endl;
        }
    }
    return res;
}

int main() {
    size_t m, n;

    ifstream input("input.txt");
    ofstream output("output.txt");

    input >> m >> n;

    short *arr = new short[m * n];
    readMatrix(input, arr, m, n);

//    printMatrix(arr, m, n);
//    cout << endl;

    output << getLongestWay(arr, m, n);
//    printMatrix(arr, m, n);

    delete[] arr;

    return 0;
}