import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.Queue;

public class Main {
	InputStream is;
	PrintWriter out;
	String INPUT = "";

	void solve() {
		int n = ni(), m = ni();
		char[][] s = new char[n][];
		for(int i = 0; i < n; i++) {
			s[i] = ns(m);
		}
		int max = Integer.MIN_VALUE;
		for(int i = 0; i < n; i++) {
			for(int j = 0; j < m; j++) {
				if(s[i][j] == '.') {
					for(int k = 0; k < n; k++) {
						for(int l = 0; l < m; l++) {
							if((i == k && j == l) || s[k][l] == '#') continue;
							int ret = minimumMove(i, j, k, l, s);
							if(ret == Integer.MAX_VALUE) continue;
							max = Math.max(ret, max);
						}
					}
				}
			}
		}
		out.println(max);
	}
	
	public int minimumMove(int startX, int startY, int endX, int endY, char[][] s) {
		int n = s.length;
		int m = s[0].length;
		char[][] t = new char[n][m];
		for(int i = 0; i < n; i++) {
			for(int j = 0; j < m; j++) {
				t[i][j] = s[i][j];
			}
		}
		Queue<int[]> q = new LinkedList<>();
		q.add(new int[] {startX, startY, 0});
		while(!q.isEmpty()) {
			int[] v = q.poll();
			if(v[0] == endX && v[1] == endY) return v[2];
			if(v[0] - 1 >= 0 && t[v[0] - 1][v[1]] == '.') {
				q.add(new int[] {v[0] - 1, v[1], v[2] + 1});
				t[v[0] - 1][v[1]] = '#';
			}
			if(v[0] + 1 < n && t[v[0] + 1][v[1]] == '.') {
				q.add(new int[] {v[0] + 1, v[1], v[2] + 1});
				t[v[0] + 1][v[1]] = '#';
			}
			if(v[1] - 1 >= 0 && t[v[0]][v[1] - 1] == '.') {
				q.add(new int[] {v[0], v[1] - 1, v[2] + 1});
				t[v[0]][v[1] - 1] = '#';
			}
			if(v[1] + 1 < m && t[v[0]][v[1] + 1] == '.') {
				q.add(new int[] {v[0], v[1] + 1, v[2] + 1});
				t[v[0]][v[1] + 1] = '#';
			}
		}
		return Integer.MAX_VALUE;
	}

	void run() throws Exception {
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);

		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if (!INPUT.isEmpty())
			tr(System.currentTimeMillis() - s + "ms");
	}

	public static void main(String[] args) throws Exception {
		new Main().run();
	}

	private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;

	private int readByte() {
		if (lenbuf == -1)
			throw new InputMismatchException();
		if (ptrbuf >= lenbuf) {
			ptrbuf = 0;
			try {
				lenbuf = is.read(inbuf);
			} catch (IOException e) {
				throw new InputMismatchException();
			}
			if (lenbuf <= 0)
				return -1;
		}
		return inbuf[ptrbuf++];
	}

	private boolean isSpaceChar(int c) {
		return !(c >= 33 && c <= 126);
	}

	private int skip() {
		int b;
		while ((b = readByte()) != -1 && isSpaceChar(b))
			;
		return b;
	}

	private double nd() {
		return Double.parseDouble(ns());
	}

	private char nc() {
		return (char) skip();
	}

	private String ns() {
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while (!(isSpaceChar(b))) { // when nextLine, (isSpaceChar(b) && b != ' ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	private char[] ns(int n) {
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while (p < n && !(isSpaceChar(b))) {
			buf[p++] = (char) b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}

	private char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for (int i = 0; i < n; i++)
			map[i] = ns(m);
		return map;
	}

	private int[] na(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = ni();
		return a;
	}

	private int ni() {
		int num = 0, b;
		boolean minus = false;
		while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
			;
		if (b == '-') {
			minus = true;
			b = readByte();
		}

		while (true) {
			if (b >= '0' && b <= '9') {
				num = num * 10 + (b - '0');
			} else {
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	private long nl() {
		long num = 0;
		int b;
		boolean minus = false;
		while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
			;
		if (b == '-') {
			minus = true;
			b = readByte();
		}

		while (true) {
			if (b >= '0' && b <= '9') {
				num = num * 10 + (b - '0');
			} else {
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	private static void tr(Object... o) {
		System.out.println(Arrays.deepToString(o));
	}
}

Measured memory_bytes: 345700