#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
#define INF 1000000000
using ll=long long;
int n,m,x[100010];
ll wsum[100010],a[100010],w[100010];
bool f(ll c){
if(c==0) return true;
vector<ll> in(n,0),ic(n,0),out(n,0),oc(n,0),cost(n,0);
ll sum=0,cnt=0;
rep(i,m){in[x[i]]+=w[i];ic[x[i]]++;out[min(n-1,x[i]+(int)(w[i]/c))]+=w[i]%c;oc[min(n-1,x[i]+(int)(w[i]/c))]++;}
rep(i,n){sum+=in[i];cnt+=ic[i]-oc[i];cost[i]+=sum;sum-=out[i]+c*cnt;}
in.assign(n,0);ic.assign(n,0);out.assign(n,0);oc.assign(n,0);sum=0,cnt=0;
rep(i,m){in[x[i]]+=w[i];ic[x[i]]++;out[max(0,x[i]-(int)(w[i]/c))]+=w[i]%c;oc[max(0,x[i]-(int)(w[i]/c))]++;}
for(int i=n-1;i>=0;i--){sum+=in[i];cnt+=ic[i]-oc[i];cost[i]+=sum;sum-=out[i]+c*cnt;}
rep(i,n)if(cost[i]-wsum[i]>=a[i])return false;
return true;
}
signed main(){
    cin>>n>>m;
    rep(i,n)cin>>a[i];
    rep(i,m){cin>>x[i]>>w[i];x[i]--;wsum[x[i]]+=w[i];}
    rep(i,n)if(wsum[i]>=a[i]){cout<<-1<<endl;return 0;}
    ll sum=0,lw=0,hi=INF,ans=INF;
    rep(i,m) sum+=w[i];
    while(lw<=hi){
        ll M=(lw+hi)/2;
        if(f(M)){
            ans=M;hi=M-1;
        }else lw=M+1;
    }
    cout<<ans<<endl;
}