#include <bits/stdc++.h>
const int maxn = 500000 + 10;
int n, S, T;
std::vector<int> e[maxn];
struct Subtask1
{
	int ans, f[maxn], g[maxn], pos[maxn];
	std::vector<std::pair<int, int>> F[maxn];
	std::vector<int> pre[maxn], suf[maxn];
	inline void dfs1(int u, int fr)
	{
		for (int v : e[u])
			if (v != fr)
				dfs1(v, u), F[u].push_back(std::make_pair(f[v], v));
		std::sort(F[u].begin(), F[u].end(), std::greater<std::pair<int, int>>());
		for (int i = 0; i < (int)F[u].size(); i++)
			f[u] = std::max(f[u], F[u][i].first + i + 1);
	}
	inline void dfs2(int u, int fr)
	{
		if (fr)
			F[u].push_back(std::make_pair(g[fr], fr));
		std::sort(F[u].begin(), F[u].end(), std::greater<std::pair<int, int>>());
		for (int i = 0; i < (int)F[u].size(); i++)
			if (F[u][i].second != fr)
				pos[F[u][i].second] = i;
		int m = (int)F[u].size();
		pre[u].resize(m);
		suf[u].resize(m);
		for (int i = 0; i < m; i++)
			pre[u][i] = suf[u][i] = F[u][i].first + i + 1;
		for (int i = 1; i < m; i++)
			pre[u][i] = std::max(pre[u][i], pre[u][i - 1]);
		for (int i = m - 2; ~i; i--)
			suf[u][i] = std::max(suf[u][i], suf[u][i + 1]);
		ans = std::min(ans, suf[u][0]);
		for (int v : e[u])
			if (v != fr)
			{
				g[u] = 0;
				if (pos[v] > 0)
					g[u] = std::max(g[u], pre[u][pos[v] - 1]);
				if (pos[v] < m - 1)
					g[u] = std::max(g[u], suf[u][pos[v] + 1] - 1);
				dfs2(v, u);
			}
	}
	inline void solve(void)
	{
		dfs1(1, 0);
		ans = f[1];
		dfs2(1, 0);
		printf("%d\n", ans);
	}
} S1;
struct Subtask2
{
	int a[maxn], m, f[maxn], pre[maxn], fa[maxn];
	std::vector<int> F[maxn];
	bool tag[maxn];
	inline void dfs(int u, int fr)
	{
		fa[u] = fr;
		for (int v : e[u])
			if (v != fr)
				dfs(v, u);
	}
	inline int dfs_pre(int u, int fr)
	{
		std::vector<int> F;
		int res = 0;
		for (int v : e[u])
			if (v != fr)
				F.push_back(dfs_pre(v, u));
		std::sort(F.begin(), F.end(), std::greater<int>());
		for (int i = 0; i < (int)F.size(); i++)
			res = std::max(res, F[i] + i + 1);
		return res;
	}
	inline bool check(int x)
	{
		int lpos = 1, rpos = m;
		for (int t = x; lpos <= m; t--, lpos++)
		{
			if (t < f[lpos])
			{
				lpos--;
				break;
			}
			if (t == f[lpos])
				t -= pre[lpos];
		}
		for (int t = x; rpos >= 1; t--, rpos--)
		{
			if (t < f[rpos])
			{
				rpos++;
				break;
			}
			if (t == f[rpos])
				t -= pre[rpos];
		}
		return rpos - lpos <= 1;
	}
	inline void solve(void)
	{
		dfs(T, 0);
		for (int u = S; u; u = fa[u])
			a[++m] = u, tag[u] = true;
		for (int i = 1; i <= m; i++)
		{
			int u = a[i];
			for (int v : e[u])
				if (!tag[v])
					F[i].push_back(dfs_pre(v, u));
			std::sort(F[i].begin(), F[i].end(), std::greater<int>());
			for (int j = 0; j < (int)F[i].size(); j++)
				if (F[i][j] + j + 1 >= f[i])
					f[i] = F[i][j] + j + 1, pre[i] = j + 1;
		}
		int l = 0, r = n;
		while (l < r)
		{
			int mid = (l + r) >> 1;
			if (check(mid))
				r = mid;
			else
				l = mid + 1;
		}
		printf("%d\n", l);
	}
} S2;
signed main()
{
	scanf("%d%d%d", &n, &S, &T);
	for (int i = 1, u, v; i < n; i++)
		scanf("%d%d", &u, &v), e[u].push_back(v), e[v].push_back(u);
	S1.solve();
	S2.solve();
	return 0;
}