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;
//        }

        // 全探索してみる
        int[] dx = {0, 0, 1, -1};
        int[] dy = {1, -1, 0, 0};
        int[][] cost = new int[h][w];
        int maxCost = -1;
        for(int y = 0;y < h;y++) {
            for(int x = 0;x < w;x++) {
                if(map[y][x] == '#') continue;
                Point start = new Point(x, y);
                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;
                        }
                    }
                }
                
                for(int y2 = 0;y2 < h;y2++) {
                    for(int x2 = 0;x2 < w;x2++) {
                        if(map[y2][x2] == '.' && maxCost < cost[y2][x2]) maxCost = cost[y2][x2];
                    }
                }
            }
        }

        PrintWriter out =  new PrintWriter(System.out);
        out.println(maxCost);
        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());}
}

Measured memory_bytes: 26580