#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}
int n,m;
vector<int> x;
vector<ll> wsum,a,w;
bool f(ll c){
    vector<ll> cost(n,0);
    // dec
    vector<ll> in(n,0),ic(n,0);
    vector<ll> out(n,0),oc(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];
        cost[i]+=sum;
        cnt-=oc[i];
        sum-=out[i];
        sum-=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];
        cost[i]+=sum;
        cnt-=oc[i];
        sum-=out[i];
        sum-=c*cnt;
    }
    rep(i,n)if(cost[i]-wsum[i]>=a[i])return false;
    return true;
}

signed main(){
    cin>>n>>m;
    a.resize(n);w.resize(m);x.resize(m);wsum.resize(n,0);
    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=1,hi=INF,ans=INF;
    rep(i,m) sum+=w[i];
    if(*min_element(ALL(a))>=sum){
        cout<<0<<endl;
        return 0;
    }
    while(lw<=hi){
        ll mid=(lw+hi)/2;
        if(f(mid)){
            ans=mid;
            hi=mid-1;
        }else lw=mid+1;
    }
    cout<<ans<<endl;
    return 0;
}