#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
using P = pair<ll, ll>;
template<typename T>
bool chmin(T &a,T b){if(a>b){a=b;return true;}else return false;}
template<typename T>
bool chmax(T &a,T b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
ostream &operator<<(ostream &os, pair<ll, ll>&p){
  return os << p.first << " " << p.second;
}  
const ll INF = 1e5+5;
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  ll n,m;cin>>n>>m;
  vector<ll>a(n);
  rep(i,0,n)cin>>a[i];
  vector<ll>x(m),w(m);
  rep(i,0,m){
    cin>>x[i]>>w[i];x[i]--;
  }
  vector<vector<ll>>g(n);
  rep(i,0,m){
    g[x[i]].PB(i);    
  }
  vector<ll>sum(n);
  rep(i,0,m){
    sum[x[i]]+=w[i];
    if(sum[x[i]]>=a[x[i]]){
      cout<<-1<<endl;
      return 0;
    }
  }
  ll le=-1,ri=INF;
  while(ri-le>=2){
    ll mid=(le+ri)/2;
    vector<ll>mx(n);
    ll nowa=0,nowb=0;
    vector<vector<ll>>in(n),out(n);
    rep(i,0,m){
      ll k;
      if(mid==0)k=INF;
      else k=w[i]/mid;
      in[max(0LL,x[i]-k)].PB(i);
      out[min(n-1,x[i]+k)].PB(i);
    }
    rep(i,0,n){
      for(auto k:in[i]){
        //cout<<i spa k<<endl;
        nowa+=mid,nowb+=-mid*x[k]+w[k];
      }
      for(auto k:g[i]){
        //cout<<"r" spa i spa k<<endl;
        nowa-=2*mid,nowb+=2*mid*x[k];
      }
      mx[i]=nowa*i+nowb;
      for(auto k:out[i]){
        //cout<<"out" spa i spa k<<endl;
        nowa+=mid,nowb-=mid*x[k]+w[k];
      }
    }
    //cout<<mid<<endl;
    //debug(mx,n);
    bool sw=true;
    rep(i,0,n)if(mx[i]>=a[i])sw=false;
    if(sw)ri=mid;
    else le=mid;
  }
  cout<<ri<<endl;
  return 0;
}