import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Deque;
import java.util.Scanner;

public class Main {
    static final Scanner sc    = new Scanner(System.in);
    static final int     MOD   = (int) 1E9 + 7;
    static final long    INF_L = (long) 4E18;
    static final int     INF   = Integer.MAX_VALUE;
    
    static int H;
    static int W;
    
    public static void main(String[] args) {
        
        H = nint();
        W = nint();
        char[][] S = nchars2(H, W, 1, 0);
        
        long maxCost = 0;
        for (int h = 1; h <= H; h++) {
            for (int w = 1; w <= W; w++) {
                if (S[h][w] == '#') continue; 
                
                maxCost = max(maxCost, walk(S, h, w));
            }
        }
        
        System.out.println(maxCost);
    }
    
    static int[] mh = {0, 1, 0, -1};
    static int[] mw = {1, 0, -1, 0};
    
    private static long walk(char[][] s, int startH, int startW) {
        Deque<Integer> dh = new ArrayDeque<>();
        Deque<Integer> dw = new ArrayDeque<>();
        
        int[][] cost = new int[H+1][W+1];
        for (int h = 0; h <= H; h++) {
            for (int w = 0; w <= W; w++) {
                cost[h][w] = INF;
            }
        }
        
        dh.add(startH);
        dw.add(startW);
        cost[startH][startW] = 0;
        
        while (!dh.isEmpty()) {
            int ch = dh.poll();
            int cw = dw.poll();
            
            for (int i = 0; i < mh.length; i++) {
                int nh = ch + mh[i];
                int nw = cw + mw[i];
                
                if (1 <= nh && nh <= H && 1 <= nw && nw <= W && s[nh][nw] == '.' && cost[nh][nw] == INF) {
                    dh.add(nh);
                    dw.add(nw);
                    cost[nh][nw] = cost[ch][cw] + 1;
                }
            }
        }
        
        long maxCost = 0;
        for (int h = 1; h <= H; h++) {
            for (int w = 1; w <= W; w++) {
                if (cost[h][w] == INF) continue;
                maxCost = max(maxCost, cost[h][w]);
            }
        }
        
        return maxCost;
    }

    @Deprecated
    static int nint() {
        return sc.nextInt();
    }

    @Deprecated
    static int[] nints(int N) {
        return nints(N, 0, 0);
    }

    @Deprecated
    private static int[] nints(int N, int padL, int padR) {
        int[] a = new int[padL + N + padR];
        for (int i = 0; i < N; i++)
            a[padL + i] = nint();
        return a;
    }

    static long nlong() {
        return sc.nextLong();
    }

    static long[] nlongs(int N) {
        return nlongs(N, 0, 0);
    }

    static long[] nlongs(int N, int padL, int padR) {
        long[] a = new long[padL + N + padR];
        for (int i = 0; i < N; i++)
            a[padL + i] = nlong();
        return a;
    }

    static double ndouble() {
        return sc.nextDouble();
    }

    static double[] ndoubles(int N) {
        return ndoubles(N, 0, 0);
    }

    static double[] ndoubles(int N, int padL, int padR) {
        double[] d = new double[N + padL + padR];
        for (int i = 0; i < N; i++) {
            d[padL + i] = ndouble();
        }
        return d;
    }

    static String nstr() {
        return sc.next();
    }

    static char[] nchars() {
        return sc.next().toCharArray();
    }

    static char[] nchars(int padL, int padR) {
        char[] temp = sc.next().toCharArray();
        char[] ret = new char[temp.length + padL + padR];
        System.arraycopy(temp, 0, ret, padL, temp.length);
        return ret;
    }

    static char[][] nchars2(int H, int W) {
        return nchars2(H, W, 0, 0);
    }

    static char[][] nchars2(int H, int W, int padLU, int padRD) {
        char[][] a2 = new char[H + padLU + padRD][W + padLU + padRD];
        for (int i = 0; i < H; i++)
            System.arraycopy(nchars(), 0, a2[padLU + i], padLU, W);
        return a2;
    }

    static long min(long... ls) {
        return Arrays.stream(ls).min().getAsLong();
    }

    static long max(long... ls) {
        return Arrays.stream(ls).max().getAsLong();
    }

    static long abs(long a) {
        return Math.abs(a);
    }
}


Measured memory_bytes: 48500