import java.io.IOException;
import java.io.InputStream;
import java.util.*;
import java.lang.*;


public class Main{
	static final long MOD = 1000000007;
	public static void main(String[] args) {
		FastScanner sc = new FastScanner();
		int h = sc.nextInt();
		int w =sc.nextInt();
		String[] s = new String[h + 2];
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < w + 2; i++) {
			sb.append('#');
		}
		s[0]= sb.toString();
		s[h + 1] = sb.toString();
		for (int i = 1; i <= h; i++) {
			s[i] = '#' + sc.next() + '#';
		}
		int max = 0;
		int[] dir = {-1, 0, 1, 0};
		for (int i = 1; i <= h; i++) {
			for (int j = 1; j <= w; j++) {
				if (s[i].charAt(j) == '#') {
					continue;
				}
				int tmpmax = 0;
				int[][] count = new int[h + 2][w + 2];
				for (int k = 0; k < h + 2; k++) {
					for (int l = 0; l < w + 2; l++) {
						count[k][l] = -1;
					}
				}
				ArrayDeque<Integer> queue = new ArrayDeque<>();
				count[i][j] = 0;
				queue.add(i * 100 + j);
				while (!queue.isEmpty()) {
					int tmp = queue.remove();
					for (int k = 0; k < 4; k++) {
						int x = tmp / 100 + dir[k];
						int y = tmp % 100 + dir[(k + 1) % 4];
						if (s[x].charAt(y) != '#' && count[x][y] == -1) {
							count[x][y] = count[tmp / 100][tmp % 100] + 1;
							tmpmax = Math.max(tmpmax, count[x][y]);
							queue.add(x * 100 + y);
						}
					}
				}
				max = Math.max(tmpmax,max);
			}
		}
		System.out.println(max);
	}
}

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 || Integer.MAX_VALUE < nl) {
			throw new NumberFormatException();
		}
		return (int) nl;
	}
	public double nextDouble() {
		return Double.parseDouble(next());
	}
}
