#include <bits/stdc++.h> #pragma GCC optimize("Ofast") using namespace std; using std::cout; using std::cin; using std::endl; using ll=long long; using ld=long double; ll ILL=2167167167167167167; const int INF=2100000000; const ll mod=1e9+7; #define rep(i,a) for (int i=0;i<a;i++) #define all(p) p.begin(),p.end() template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>; template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();} template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();} template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;} template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;} template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());} template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});} void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";} template<class T> void vec_out(vector<T> &p){for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";} template<class T> T min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;} template<class T> T max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;} void solve(); // oddloop int main() { ios::sync_with_stdio(false); cin.tie(nullptr); int t=1; //cin>>t; rep(i,t) solve(); } void solve(){ int N,M; cin>>N>>M; vector<ll> H(N); rep(i,N) cin>>H[i]; vector<vector<int>> A(N),B(N); rep(i,M){ int a,b; cin>>a>>b; a--,b--; A[a].push_back(b); B[N-1-b].push_back(N-1-a); } rep(roop,2){ if(roop){ swap(A,B); reverse(all(H)); } vector<ll> dp1(N,-ILL),dp2(N,-ILL); dp1[0]=0; rep(i,N){ for(auto x:A[i]){ if(H[i]<H[x]){ chmax(dp2[x],dp1[i]+H[x]-H[i]); }else{ chmax(dp1[x],max(dp1[i],dp2[i])); } } } cout<<max(-1ll,max(dp1[N-1],dp2[N-1]))<<"\n"; } }