#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;
vector<vector<ll>> w(100001);

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,n){
            for(auto x:w[i]){
                in[i].push_back(x);
                out[min(n-1,i+(int)(x/c))].push_back(x%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,n){
            for(auto x:w[i]){
                in[i].push_back(x);
                out[max(0,i-(int)(x/c))].push_back(x%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){
        ll sum=0;
        for(auto x:w[i]) sum+=x;
        if(cost[i]-sum>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(n);
    rep(i,n) cin>>a[i];
    rep(i,m){
        int j,x;cin>>j>>x;j--;
        w[j].push_back(x);
    }

    rep(i,n){
        ll sum=0;
        for(auto x:w[i]) sum+=x;
        if(sum>a[i]){
            cout<<-1<<endl;
            return 0;
        }
    }

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

    ll lw=1,hi=LINF,ans=LINF;
    while(lw<=hi){
        ll mid=(lw+hi)/2;
        if(f(mid)){
            chmin(ans,mid);
            hi=mid-1;
        }else{
            lw=mid+1;
        }
    }
    cout<<ans<<endl;
    return 0;
}