#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<pll,ll> P;
int main(){
  const ll INF=1000000000;
  ll i,n,m,s,t;
  cin >> n >> m >> s >> t;
  s--,t--;
  vector<ll> p(n);
  for(i=0;i<n;i++){
    cin >> p[i];
  }
  vector<vector<ll> > g(n);
  for(i=0;i<m;i++){
    ll a,b;
    cin >> a >> b;
    a--,b--;
    g[a].push_back(b);
    g[b].push_back(a);
  }
  vector<ll> ds(n,-INF),ns(n,0);
  ds[s]=0,ns[s]=1;
  priority_queue<P> que;
  que.push(P(pll(p[s],0),s));
  while(!que.empty()){
    P p1=que.top();
    que.pop();
    ll u=p1.first.first,v=p1.first.second;
    ll r=p1.second;
    if(ds[r]!=v){
      continue;
    }
    for(i=0;i<g[r].size();i++){
      ll us=g[r][i];
      if(u>p[us]){
        if(ds[us]<ds[r]+1){
          ds[us]=ds[r]+1;
          que.push(P(pll(p[us],ds[us]),us));
        }
      }
      else{
        if(ds[us]<ds[r]){
          ds[us]=ds[r];
          que.push(P(pll(u,ds[us]),us));
        }
      }
    }
    ns[r]=1;
  }
  cout << ds[t] << endl;
}