#include<bits/stdc++.h>
using namespace std;
//#pragma GCC optimize("Ofast")

#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
void solve(ll n,ll m,vl v,vvl g){
    vvl dp(n,vl(2,-INF));dp[0][0]=0;
    rep(i,n){
        for(auto p:g[i]){
            if(v[i]-v[p]>0)chmax(dp[i][1],dp[p][0]+v[i]-v[p]);
            else {
                chmax(dp[i][0],dp[p][0]);
                chmax(dp[i][0],dp[p][1]);
            }
        }
    }
    ll f=max(dp[n-1][0],dp[n-1][1]);
    cout << (f<0? -1:f) << endl;
}
int main(){
    ll n,m;cin >> n >> m;
    vl v(n);rep(i,n)cin >> v[i];
    vvl g(n),rg(n);
    rep(i,m){
        ll a,b;cin >> a >> b;a--;b--;
        g[b].emplace_back(a);
        rg[n-1-a].emplace_back(n-1-b);
    }
    solve(n,m,v,g);
    rev(all(v));
    solve(n,m,v,rg);
}