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 main()
{
	auto N = RD;
	auto u = RD-1;
	auto v = RD-1;
	auto edges = new long[][](N);
	foreach (i; 0..N-1)
	{
		auto A = RD-1;
		auto B = RD-1;
		edges[A] ~= B;
		edges[B] ~= A;
	}

	auto rank = new long[](N);
	void dfs(long pos, long par)
	{
		foreach (to; edges[pos])
		{
			if (to == par) continue;
			rank[to] = rank[pos] + 1;
			dfs(to, pos);
		}
	}
	dfs(v, -1);
	auto rank2 = new long[](N);
	void dfs2(long pos, long par)
	{
		foreach (to; edges[pos])
		{
			if (to == par) continue;
			rank2[to] = rank2[pos] + 1;
			debug writeln(rank2[to], ":", rank[to]);
			if (rank2[to] < rank[to])
				dfs2(to, pos);
		}
	}
	dfs2(u, -1);
	
	long ans;
	foreach (i; 0..N)
	{
		if (rank2[i] <= rank[i])
		{
			auto d = rank2[i] - rank[i];
			ans.chmax(rank2[i]+d/2);
		}
	}
	debug writeln(rank);
	debug writeln(rank2);

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