//#include <bits/stdc++.h>
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
//#include "boost/multiprecision/cpp_int.hpp"
//typedef boost::multiprecision::cpp_int ll;
typedef long double dd;
#define i_7 (ll)(1E9+7)
//#define i_7 998244353
#define i_5 i_7-2
ll mod(ll a){
    ll c=a%i_7;
    if(c>=0)return c;
    return c+i_7;
}
typedef pair<ll,ll> l_l;
typedef pair<dd,dd> d_d;
ll inf=(ll)1E16;
#define rep(i,l,r) for(ll i=l;i<=r;i++)
#define pb push_back
ll max(ll a,ll b){if(a<b)return b;else return a;}
ll min(ll a,ll b){if(a>b)return b;else return a;}
void Max(ll &pos,ll val){pos=max(pos,val);}//Max(dp[n],dp[n-1]);
void Min(ll &pos,ll val){pos=min(pos,val);}
void Add(ll &pos,ll val){pos=mod(pos+val);}
dd EPS=1E-9;
#define fastio ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define fi first
#define se second
///////////////////////////

#define N 100005
ll n,m;
ll a[N];
ll x[N],w[N];
bool check(){
    ll sum[n];memset(sum,0,sizeof(sum));
    rep(i,0,m-1){
        sum[x[i]]+=w[i];
    }
    rep(i,0,n-1){
        if(sum[i]>=a[i])return true;
    }
    return 0;
}
bool zerocheck(){
    ll sum=0;
    rep(i,0,m-1)sum+=w[i];
    rep(i,0,n-1){
        if(sum>=a[i])return false;
    }
    return true;
}

int main(){fastio
    cin>>n>>m;
    rep(i,0,n-1)cin>>a[i];
    rep(i,0,m-1){
        cin>>x[i]>>w[i];x[i]--;
    }
    if(check()){
        cout<<-1<<endl;return 0;
    }
    if(zerocheck()){
        cout<<0<<endl;return 0;
    }
    ll ok=inf,ng=0;
    while(abs(ok-ng)>1){
        ll mid=(ok+ng)/2;
        ll imo[n],umo[n];memset(imo,0,sizeof(imo));memset(umo,0,sizeof(umo));
        ll iml[n],uml[n];memset(iml,0,sizeof(iml));memset(uml,0,sizeof(uml));
        rep(i,0,m-1){
            if(x[i]+1<=n-1)imo[x[i]+1]-=mid;
            ll t=w[i]/mid;
            if(x[i]+1+t<=n-1)imo[x[i]+1+t]+=mid;
            if(x[i]+1+t<=n-1)umo[x[i]+1+t]+=mid*t;
            umo[x[i]]+=w[i];
            if(x[i]+t+1<=n-1)umo[x[i]+t+1]-=w[i];
            
            if(x[i]-1>=0)iml[x[i]-1]-=mid;
            t=w[i]/mid;
            if(x[i]-1-t>=0){
                iml[x[i]-1-t]+=mid;
                uml[x[i]-1-t]+=mid*t;
                uml[x[i]-t-1]-=w[i];
            }
            if(x[i]-1>=0)uml[x[i]-1]+=w[i];
            
        }
        
        rep(i,1,n-1)imo[i]+=imo[i-1];
        rep(i,0,n-1)umo[i]+=imo[i];
        rep(i,1,n-1)umo[i]+=umo[i-1];
        for(ll i=n-2;i>=0;i--)iml[i]+=iml[i+1];
        for(ll i=n-1;i>=0;i--)uml[i]+=iml[i];
        for(ll i=n-2;i>=0;i--)uml[i]+=uml[i+1];
        ll sum[n];memset(sum,0,sizeof(sum));
        rep(i,0,n-1)sum[i]=umo[i]+uml[i];
        bool flag=true;
        rep(i,0,n-1){
            if(sum[i]>=a[i]){flag=false;break;}
        }
        if(flag)ok=mid;
        else ng=mid;
    }
    cout<<ok<<endl;
    
    return 0;
}