#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 debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
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<ll> a,w;
vector<int> x;
vector<ll> wsum;

bool f(ll c){
    vector<ll> cost(n,0);
    // dec
    {
        vector<vector<ll>> in(n);
        vector<vector<ll>> out(n);
        ll sum=0,cnt=0;
        rep(i,m){
            in[x[i]].push_back(w[i]);
            out[min(n-1,x[i]+(int)(w[i]/c))].push_back(w[i]%c);
        }
        rep(i,n){
            for(auto x:in[i]){
                sum+=x;
                cnt++;
            }
            cost[i]+=sum;
            for(auto x:out[i]){
                cnt--;
                sum-=x;
            }
            sum-=c*cnt;
        }
    }
    //inc
    {
        vector<vector<ll>> in(n);
        vector<vector<ll>> out(n);
        ll sum=0,cnt=0;
        rep(i,m){
            in[x[i]].push_back(w[i]);
            out[max(0,x[i]-(int)(w[i]/c))].push_back(w[i]%c);
        }
        for(int i=n-1;i>=0;i--){
            for(auto x:in[i]){
                sum+=x;
                cnt++;
            }
            cost[i]+=sum;
            for(auto x:out[i]){
                cnt--;
                sum-=x;
            }
            sum-=c*cnt;
        }
    }
    // cout<<endl;
    // debug(cost);
    rep(i,n){
        if(cost[i]-wsum[i]>=a[i]){
            // cout<<c<<" false\n";
            return false;
        }
    }
    // cout<<c<<" true\n";
    return true;
}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    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){
        ll sum=0;
        if(wsum[i]>=a[i]){
            cout<<-1<<endl;
            return 0;
        }
    }

    {
        ll sum=0;
        rep(i,m) sum+=w[i];
        if(*min_element(ALL(a))>=sum){
            cout<<0<<endl;
            return 0;
        }
    }

    ll lw=1,hi=INF,ans=INF;
    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;
}