import java.awt.Point;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Deque;
import java.util.NoSuchElementException;

public class Main implements Runnable {
    public static void main(String[] args) {
        new Thread(null,new Main(), "" ,Runtime.getRuntime().maxMemory()).start();
    }

    // メイン処理
    public void run() {
        FastScanner sc = new FastScanner();
        int h = sc.nextInt();
        int w = sc.nextInt();
        char[][] map = new char[h][w];
        for(int i = 0;i < h;i++) map[i] = sc.next().toCharArray();

        // とりあえず適当に.を見つける
        Point start = new Point();
        boolean finish = false;
        for(int y = 0;y < h;y++) {
            for(int x = 0;x < w;x++) {
                if(map[y][x] == '.') {
                    start.setLocation(x, y);
                    finish = true;
                    break;
                }
            }
            if(finish) break;
        }

        // 適当な点から一番遠い点を導出してその点から一番遠い点を導出すればいい気がする。
        int[] dx = {0, 0, 1, -1};
        int[] dy = {1, -1, 0, 0};
        int[][] cost = new int[h][w];
        for(int i = 0;i < h;i++) Arrays.fill(cost[i], Integer.MAX_VALUE - 100);
        cost[start.y][start.x] = 0;
        boolean[][] footprints = new boolean[h][w];
        footprints[start.y][start.x] = true;
        Deque<Point> que = new ArrayDeque<>();
        que.offer(start);
        while(!que.isEmpty()) {
            Point p = que.poll();
            for(int i = 0;i < 4;i++) {
                int nextX = p.x + dx[i];
                int nextY = p.y + dy[i];
                if(nextX < 0 || nextX >= w || nextY < 0 || nextY >= h || map[nextY][nextX] == '#') continue;
                if(cost[p.y][p.x] + 1 < cost[nextY][nextX]) cost[nextY][nextX] = cost[p.y][p.x] + 1; 
                if(!footprints[nextY][nextX]) {
                    que.offer(new Point(nextX, nextY));
                    footprints[nextY][nextX] = true;
                }
            }
        }
        
        int maxCost = -1;
        int nextStartX = 0;
        int nextStartY = 0;
        for(int y = 0;y < h;y++) {
            for(int x = 0;x < w;x++) {
                if(map[y][x] == '.' && maxCost < cost[y][x]) {
                    maxCost = cost[y][x];
                    nextStartX = x;
                    nextStartY = y;
                }
            }
        }

        start = new Point(nextStartX, nextStartY);
        for(int i = 0;i < h;i++) Arrays.fill(cost[i], Integer.MAX_VALUE - 100);
        cost[start.y][start.x] = 0;
        footprints = new boolean[h][w];
        footprints[start.y][start.x] = true;
        que.clear();
        que.offer(start);
        while(!que.isEmpty()) {
            Point p = que.poll();
            for(int i = 0;i < 4;i++) {
                int nextX = p.x + dx[i];
                int nextY = p.y + dy[i];
                if(nextX < 0 || nextX >= w || nextY < 0 || nextY >= h || map[nextY][nextX] == '#') continue;
                if(cost[p.y][p.x] + 1 < cost[nextY][nextX]) cost[nextY][nextX] = cost[p.y][p.x] + 1; 
                if(!footprints[nextY][nextX]) {
                    que.offer(new Point(nextX, nextY));
                    footprints[nextY][nextX] = true;
                }
            }
        }
        
        maxCost = -1;
        nextStartX = 0;
        nextStartY = 0;
        for(int y = 0;y < h;y++) {
            for(int x = 0;x < w;x++) {
                if(map[y][x] == '.' && maxCost < cost[y][x]) {
                    maxCost = cost[y][x];
                    nextStartX = x;
                    nextStartY = y;
                }
            }
        }

        start = new Point(nextStartX, nextStartY);
        for(int i = 0;i < h;i++) Arrays.fill(cost[i], Integer.MAX_VALUE - 100);
        cost[start.y][start.x] = 0;
        footprints = new boolean[h][w];
        footprints[start.y][start.x] = true;
        que.clear();
        que.offer(start);
        while(!que.isEmpty()) {
            Point p = que.poll();
            for(int i = 0;i < 4;i++) {
                int nextX = p.x + dx[i];
                int nextY = p.y + dy[i];
                if(nextX < 0 || nextX >= w || nextY < 0 || nextY >= h || map[nextY][nextX] == '#') continue;
                if(cost[p.y][p.x] + 1 < cost[nextY][nextX]) cost[nextY][nextX] = cost[p.y][p.x] + 1; 
                if(!footprints[nextY][nextX]) {
                    que.offer(new Point(nextX, nextY));
                    footprints[nextY][nextX] = true;
                }
            }
        }
        
        int ans = -1;
        for(int y = 0;y < h;y++) {
            for(int x = 0;x < w;x++) {
                if(map[y][x] == '.' && ans < cost[y][x]) ans = cost[y][x];
            }
        }
        
        PrintWriter out =  new PrintWriter(System.out);
        out.println(ans);
        out.flush();
    }

    // 以下、ユーティリティ
    long nCm(long n, long m) {
        if(n < m) return 0l;
        long c = 1l;
        m = (n - m < m ? n - m : m);
        for(long ns = n - m + 1, ms = 1;ms <= m;ns ++, ms++) {
            c *= ns;
            c /= ms;
        }
        return c;
    }

    long modnCm(int n, int m, int mod) {
        if(n < m) return 0l;
        long upper = 1l;
        long downer = 1l;
        m = (n - m < m ? n - m : m);
        for(long ns = n - m + 1, ms = 1;ms <= m;ns ++, ms++) {
            upper = upper * ns % mod;
            downer = downer * ms % mod;
        }
        return upper * modInv(downer, mod) % mod;
    }

    // mod m での a の逆元 を計算する
    long modInv(long a, long m) {
        long b = m, u = 1, v = 0;
        while(b > 0) {
            long t = a / b;
            a -= t * b;
            long tmp = a;
            a = b;
            b = tmp;
            u -= t * v;
            tmp = u;
            u = v;
            v = tmp;
        }
        u %= m;
        if (u < 0) u += m;
        return u;
    }

    long modPow(long a, long n, long mod) {
        long res = 1l;
        while(n > 0l) {
            if(n % 2l == 1l) res = res * a % mod;
            a = a * a % mod;
            n /= 2l;
        }
        return res;
    }

}

class UnionFind {
    private int[] parent;

    UnionFind(int n) {
        parent = new int[n];
        for(int i = 0;i < n;i++) parent[i] = i;
    }

    public int rootOf(int x) {
        if(parent[x] == x) return x;
        else return parent[x] = rootOf(parent[x]);
    }

    public boolean same(int x, int y) {
        return rootOf(x) == rootOf(y);
    }

    public void unite(int x, int y) {
        x = rootOf(x);
        y = rootOf(y);
        if(x == y) return;
        parent[x] = y;
        return;
    }
}

class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() { return Double.parseDouble(next());}
}