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

struct Dsu{
  vector<int> p;
  vector<int> r;

  Dsu() = delete;
  Dsu(int n){
    p.resize(n); for (int i=n; i--;) p[i]=i;
    r.resize(n);
  }

  int root(int a){
    return p[a]==a? a: p[a]=root(p[a]);
  }

  void merge(int a,int b){
    a=root(a);
    b=root(b);
    if (r[a]<r[b]) swap(a,b);
    p[b]=a;
    r[a]++;
  }
};

int main(){
  int n,m; cin>>n>>m;

  vector<int64_t> h(n);
  vector<vector<int>> e(n);

  for (int i=0; i<n; i++){
    cin>>h[i];
  }
  for (int i=0; i<m; i++){
    int a,b; cin>>a>>b; --a,--b;
    e[a].push_back(b);
    e[b].push_back(a);
  }
  assert(!(cin>>m));

  vector<pair<int64_t,int>> by_height(n);
  for (int i=0; i<n; i++) by_height[i]={h[i],i};
  sort(by_height.begin(),by_height.end());

  int64_t impossible=(by_height[n-1].first-by_height[0].first)+1;
  int64_t best=impossible;

  for (int i=0; i<n; i++){
    if ((i>0 and by_height[i]==by_height[i-1])) continue;
    if (by_height[i].first>max(h[0],h[1])) break;
    Dsu d(n);
    for (auto jt=by_height.begin()+i; jt!=by_height.end(); ++jt){
      for (int k: e[jt->second]){
        if (by_height[i].first<=h[k] and h[k]<=jt->first){
          d.merge(jt->second, k);
        }
      }
      if (d.root(0)==d.root(1)){
        best=min(best,jt->first-by_height[i].first);
        break;
      }
    }
  }

  if (best==impossible){
    cout<<"impossible"<<endl;
  }else{
    cout<<best<<endl;
  }
}