#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using ulint = unsigned long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
template<typename T> istream &operator>>(istream &is, vector<T> &vec){ for (auto &v : vec) is >> v; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec){ os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << "," << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template<typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template<typename T> bool mjjj(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool mmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }
template<typename T1, typename T2> pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template<typename T1, typename T2> pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/tag_and_trait.hpp>
using namespace __gnu_pbds; // find_by_order(), order_of_key()
template<typename TK> using pbds_set = tree<TK, null_type, less<TK>, rb_tree_tag, tree_order_statistics_node_update>;
template<typename TK, typename TV> using pbds_map = tree<TK, TV, less<TK>, rb_tree_tag, tree_order_statistics_node_update>;
*/

template< typename T >
struct edge {
int src, to;
T cost;
edge(int to, T cost) : src(-1), to(to), cost(cost) {}
edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
edge &operator=(const int &x) {
to = x;
return *this;
}
operator int() const { return to; }
};
template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;
lint lcm(lint a, lint b);
lint gcd(lint a, lint b);
#define INF 1e+9
#define MAX_V 100000

// <最短距離, 頂点の番号>
using P = pair<lint, lint>;

lint V;
vector<edge<lint>> G[MAX_V];
lint d[MAX_V];

void dijkstra(int s) {
    priority_queue<P, vector<P>, greater<P> > que;
    fill(d, d+V, INF);
    d[s] = 0;
    que.push(P(0, s));

    while (!que.empty()) {
        P p = que.top();
        que.pop();
        lint v = p.second;
        if (d[v] < p.first) continue;

        for (lint i=0; i<G[v].size(); ++i) {
            edge<lint> e = G[v][i];
            if (d[e.to] > d[v] + e.cost) {
                d[e.to] = d[v] + e.cost;
                que.push(P(d[e.to], e.to));
            }
        }
    }
}
int main()
{
    cin >> V;
    lint u,v,ans;
    ans = 0;
    cin >> u >> v;
    for (lint i=0; i<V; ++i) {
        lint a, b;
        cin >> a >> b;
        edge<lint> e = {b, (lint)1};
        G[a].push_back(e);
    }
    lint dup = 0;
    dijkstra(v-1);
    if(d[u-1] == 2){
        REP(i,G[v-1].size()){
            REP(j,G[u-1].size()){
                if(G[v-1][i].to == G[u-1][j].to){
                    G[u-1][i].cost = INF;
                }
            }
        }
        goto skip;
    }
    else{
        for (lint i=0; i<V; ++i) {
            if(d[i] != INF)
                ans = max(d[i],ans);
        }
        cout << ans;
        return 0;
    }
    skip:
    dijkstra(u-1);
        for (lint i=0; i<V; ++i) {
            if(d[i] < INF)
                ans = max(d[i],ans);
        }
        cout << ans + 1;

}

lint gcd(lint a, lint b)
{
    if (a < b) {
        a ^= b;
        b ^= a;
        a ^= b;
    }
    
    return b ? gcd(b, a % b) : a;
}

lint lcm(lint a, lint b)
{
    return a * b / gcd(a, b);
}