#include<bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (int) n; i++)

using ll = long long;
using namespace std;
using T = tuple<ll, ll, ll>;
using P = pair<ll, ll>;

const long long INF = 1ll << 60;

template<class T>
bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}


int main() {
    ll n, m, s, t;
    cin >> n >> m >> s >> t;
    s--;
    t--;
    vector<ll> p(n);
    rep(i, n) cin >> p[i];
    vector<vector<ll>> vertex(n);
    rep(i, m) {
        ll a, b;
        cin >> a >> b;
        a--;
        b--;
        vertex[a].push_back(b);
        vertex[b].push_back(a);
    }
    
    priority_queue<T> pri;
    pri.emplace(p[s], s, 0);
    vector<ll> dp(n);
    set<P> seen;
    seen.insert({s, p[s]});
    while (!pri.empty()) {
        auto[people, now, score] = pri.top();
//        cout << people << " " << now << " " << score << endl;
        pri.pop();
        
        for (auto next:vertex[now]) {
            if (p[next] < people) {
                chmax(dp[next], score + 1);
                pri.emplace(p[next], next, score + 1);
                seen.insert({next, p[next]});
            } else if (seen.end() == seen.find({next, people})) {
                chmax(dp[next], score);
                pri.emplace(people, next, score);
                seen.insert({next, people});
            }
        }
    }
    
    cout << dp[t] << endl;
}