#include <bits/stdc++.h>
using namespace std;

#define NDEBUG
#include <cassert>


typedef long long ll;
typedef long double Double;
typedef unsigned long long ull;
typedef pair<int,int> ii;
typedef pair<ll,ll> llll;
typedef pair<double,double> dd;

typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<ii> vii;
typedef vector<vector<ii>> vvii;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<llll> vllll;
typedef vector<bool> vb;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<long double> vD;

#define sz(a)  int((a).size())
#define pb  push_back
#define eb  emplace_back
#define FOR(var,from,to) for(int var=(from);var<=(to);++var)
#define rep(var,n)  for(int var=0;var<(n);++var)
#define rep1(var,n)  for(int var=1;var<=(n);++var)
#define repC2(vari,varj,n)  for(int vari=0;vari<(n)-1;++vari)for(int varj=vari+1;varj<(n);++varj)
#define repC3(vari,varj,vark,n)  for(int vari=0;vari<(n)-2;++vari)for(int varj=vari+1;varj<(n)-1;++varj)for(int vark=varj+1;vark<(n);++vark)
#define ALL(c)  (c).begin(),(c).end()
#define RALL(c)  (c).rbegin(),(c).rend()
#define tr(i,c)  for(auto i=(c).begin(); i!=(c).end(); ++i)
#define mset(arr,val)  memset(arr,val,sizeof(arr))
#define mid(x,y) ((x)+((y)-(x))/2)
#define IN(x,a,b) ((a)<=(x)&&(x)<=(b))
#define cons make_pair
#define clamp(v,lo,hi) min(max(v,lo),hi)

template <typename T>
inline bool found(set<T> &s, T elem) { return s.find(elem) != s.end(); }
template <typename T>
inline bool found(unordered_set<T> &s, T elem) { return s.find(elem) != s.end(); }
template <typename T, typename U>
inline bool found(map<T,U> &s, U elem) { return s.find(elem) != s.end(); }
template <typename T, typename U>
inline bool found(unordered_map<T,U> &s, U elem) { return s.find(elem) != s.end(); }
template <typename T>
inline bool found(vector<T> &s, T elem) { return find(s.begin(), s.end(), elem) != s.end(); }
inline int found(string &s, string t) { return (int)s.find(t); }
inline int found(string &s, int c) { return (int)s.find(c); }

template<typename T1, typename T2> inline void amin(T1 & a, T2 const & b) { if (a>b) a=b; }
template<typename T1, typename T2> inline void amax(T1 & a, T2 const & b) { if (a<b) a=b; }
template<typename X, typename T> auto vectors(X x, T a) { return vector<T>(x, a); }
template<typename X, typename Y, typename Z, typename... Zs> auto vectors(X x, Y y, Z z, Zs... zs) { auto cont = vectors(y, z, zs...); return vector<decltype(cont)>(x, cont); }

inline ll square(ll x) { return x * x; }
inline ll gcd(ll a, ll b) { while(a) swap(a, b%=a); return b; }
template <typename T>
inline T mod(T a, T b) { return ((a % b) + b) % b; }

const int INF = 999999;

template <typename T>
void make_adjacent(int nV, vector<pair<ii,T>>& arcs, vector<vector<pair<int,T>>>& adj) {
	adj.resize(nV);
	for (pair<ii, T> p: arcs) {
		int u = p.first.first, v = p.first.second;
		assert(u != v);
		T distance = p.second;
		adj[u].push_back(make_pair(v, distance));
		adj[v].push_back(make_pair(u, distance));
	}
}

template <typename T>
void dij1(int nV, vector<vector<pair<int,T>>>& adjacent, vector<T>& d, int start) {
	d.assign(nV, INF);
	d[start] = 0;
	vector<int> prev(nV, -1);

	priority_queue<pair<T, int>> Q;
	Q.push(make_pair(0, start));

	vector<bool> visited(nV, false);

	while (!Q.empty()) {
		T du = -Q.top().first;
		int u = Q.top().second; Q.pop();
		if (visited[u]) continue;
		visited[u] = true;

		int c = adjacent[u].size();
		for (int i=0; i<c; ++i) {
			int v = adjacent[u][i].first;
			T distance = adjacent[u][i].second;
			T alt = du + distance;
			if (d[v] > alt) {
				d[v] = alt;
				prev[v] = u;
				Q.push(make_pair(-alt, v));
			}
		}
	}
}

int solve(int N, int u,int v, vi& a, vi& b) {
    vector<vector<ii>> adj(N);

    rep(i,N-1){
        adj[ a[i] ].emplace_back(b[i], 1);
        adj[ b[i] ].emplace_back(a[i], 1);
    }

    vi da(N, INF), db(N, INF);
    dij1(N, adj, da, u);
    dij1(N, adj, db, v);

    int best = db[u]-1;
    vii de(N);
    rep(i,N){
        if (db[i] <= da[i]) continue;
        amax(best, da[i]);
    }
    return  best;
}

int main() {
    int N,u,v; scanf("%d %d %d%*c", &N, &u, &v);
    --u; --v;
    vi a(N-1),b(N-1);
    rep(i,N-1){
        scanf("%d %d%*c", &a[i], &b[i]);
        a[i]--; b[i]--;
    }
    cout << solve(N,u,v,a,b) << endl;
    return 0;
}
