import java.io.*;
import java.math.*;
import java.util.*;
public class Main { 

	public static void main(String[] args) {    
		FastScanner I = new FastScanner(); //Input
		OutPut O = new OutPut(); //Output
		int R = I.nextInt();
		int C = I.nextInt();
		char[][] grid = new char[R][C];
		int ans = 0;
		for (int r = 0; r < R; r++) {
			grid[r] = I.next().toCharArray();
		}
		for (int r = 0; r<R; r++) {
			for (int c = 0; c < C; c++) {
				if (grid[r][c]=='.') {
					for (int nextr = 0; nextr<R; nextr++) {
						for (int nextc = 0; nextc<C; nextc++) {
							if (grid[nextr][nextc]=='.'&&!(nextr==r&&nextc==c)) {
								int[] start = {r,c};
								int[] end = {nextr,nextc};
								ans=Math.max(ans, BFS(grid,start,end));
							}
						}
					}
				}
			}
		}
		O.pln(ans);
	}
	public static int BFS(char[][] grid, int[] start, int[] end) {
		int len = 0;
		int R = grid.length;
		int C = grid[0].length;
		boolean[][] vis = new boolean[R][C];
		Queue<int[]> cur = new LinkedList<int[]>();
		vis[start[0]][start[1]]=true;
		cur.add(start);
		while (!cur.isEmpty()) {
			int size = cur.size();
			for (int i = 0; i < size; i++) {
				int[] curpos = cur.poll();
				int r = curpos[0];
				int c = curpos[1];
				if (r==end[0]&&c==end[1]) return len;
				if (r+1<R&&!vis[r+1][c]&&grid[r+1][c]=='.') {
					vis[r+1][c]=true;
					int[] pair = {r+1,c};
					cur.add(pair);
				}
				if (r-1>=0&&!vis[r-1][c]&&grid[r-1][c]=='.') {
					vis[r-1][c]=true;
					int[] pair = {r-1,c};
					cur.add(pair);
				}
				if (c+1<C&&!vis[r][c+1]&&grid[r][c+1]=='.') {
					vis[r][c+1]=true;
					int[] pair = {r,c+1};
					cur.add(pair);
				}
				if (c-1>=0&&!vis[r][c-1]&&grid[r][c-1]=='.') {
					vis[r][c-1]=true;
					int[] pair = {r,c-1};
					cur.add(pair);
				}
			}
			len++;
		}
		return -1;
	}
	public static long ceil(long num, long den) {long ans = num/den; if (num%den!=0) 
	ans++; return ans;}
	public static long GCD(long a, long b) {
		if (a==0||b==0) return Math.max(a,b);
		return GCD(Math.min(a, b),Math.max(a, b)%Math.min(a, b));
	}
	public static long FastExp(long base, long exp, long mod) {
		long ans=1;
		while (exp>0) {
			if (exp%2==1) ans*=base;
			exp/=2;
			base*=base;
			base%=mod;
			ans%=mod;
		}
		return ans;
	}
	public static long ModInv(long num,long mod) {return FastExp(num,mod-2,mod);}
	public static int pop(long x) { //Returns number of bits within a number
		int cnt = 0;
		while (x>0) {
			if (x%2==1) cnt++;
			x/=2;
		}
		return cnt;
	}
	static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		int nextInt() {return Integer.parseInt(next());}
		long nextLong() {return Long.parseLong(next());};
	}
	static class OutPut{
		PrintWriter w = new PrintWriter(System.out);
		void pln(double x) {w.println(x);w.flush();}
		void pln(int x) {w.println(x);w.flush();}
		void pln(long x) {w.println(x);w.flush();}
		void pln(String x) {w.println(x);w.flush();}
		void pln(char x) {w.println(x);w.flush();}
		void pln(StringBuilder x) {w.println(x);w.flush();}
		void pln(BigInteger x) {w.println(x);w.flush();}
		void p(int x) {w.print(x);w.flush();}
		void p(long x) {w.print(x);w.flush();}
		void p(String x) {w.print(x);w.flush();}
		void p(char x) {w.print(x);w.flush();}
		void p(StringBuilder x) {w.print(x);w.flush();}
		void p(BigInteger x) {w.print(x);w.flush();}
		void p(double x) {w.print(x);w.flush();}
	}
}


Measured memory_bytes: 44916