import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.bigint, std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static File _f;
void file_io(string fn) { _f = File(fn, "r"); }
static string[] s_rd;
T _RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T _RD(T = long)(File f) { while(!s_rd.length) s_rd = f.readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T[] _RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; }
T[] _RDA(T = long)(File f, T fix = 0) { auto r = f.readln.chomp.split.to!(T[]); r[] += fix; return r; }
T RD(T = long)() { if (_f.isOpen) return _RD!T(_f); else return _RD!T; }
T[] RDA(T = long)(T fix = 0) { if (_f.isOpen)  return _RDA!T(_f, fix); else return _RDA!T(fix); }

size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

void chmin(T)(ref T x, T y) { x = min(x, y); } void chmax(T)(ref T x, T y) { x = max(x, y); }
bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
long lcm(long x, long y) { return x * (y / gcd(x, y)); }

long mod = 10^^9 + 7;
//long mod = 998244353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }
void modpow(ref long x, long y) { if (!y) { x = 1; return; } auto t = x; x.modpow(y>>1); x.modm(x); if (y&1) x.modm(t); }

void main()
{
	auto H = RD;
	auto W = RD;
	auto S = new string[](H);
	foreach (i; 0..H)
		S[i] = RD!string;

	long bfs(long sy, long sx)
	{
		long[][] open = [[sy, sx]];
		auto dist = new long[][](H, W);
		foreach (i; 0..H)
			dist[i][] = long.max;
		dist[sy][sx] = 0;
		while (!open.empty)
		{
			auto nd = open.front; open.popFront;
			auto y = nd[0];
			auto x = nd[1];
			if (y != 0)
			{
				auto ny = y-1;
				auto nx = x;
				if (S[ny][nx] == '.' && dist[y][x]+1 < dist[ny][nx])
				{
					dist[ny][nx] = dist[y][x]+1;
					open ~= [ny, nx];
				}
			}
			if (y != H-1)
			{
				auto ny = y+1;
				auto nx = x;
				if (S[ny][nx] == '.' && dist[y][x]+1 < dist[ny][nx])
				{
					dist[ny][nx] = dist[y][x]+1;
					open ~= [ny, nx];
				}
			}
			if (x != 0)
			{
				auto ny = y;
				auto nx = x-1;
				if (S[ny][nx] == '.' && dist[y][x]+1 < dist[ny][nx])
				{
					dist[ny][nx] = dist[y][x]+1;
					open ~= [ny, nx];
				}
			}
			if (x != W-1)
			{
				auto ny = y;
				auto nx = x+1;
				if (S[ny][nx] == '.' && dist[y][x]+1 < dist[ny][nx])
				{
					dist[ny][nx] = dist[y][x]+1;
					open ~= [ny, nx];
				}
			}
		}
		long res;
		foreach (y; 0..H)
		{
			foreach (x; 0..W)
			{
				if (S[y][x] == '#') continue;
				res.chmax(dist[y][x]);
			}
		}
		return res;
	}

	long ans;
	foreach (y; 0..H)
	{
		foreach (x; 0..W)
		{
			if (S[y][x] == '#') continue;
			ans.chmax(bfs(y, x));
		}
	}

	writeln(ans);
	stdout.flush;
	debug readln;
}


Measured memory_bytes: 1276