#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <algorithm>
#include <set>
#include <map>
#include <bitset>
#include <cmath>
#include <functional>
#include <iomanip>
#define vll vector<ll>
#define vvvl vector<vvl>
#define vvl vector<vector<ll>>
#define VV(a, b, c, d) vector<vector<d>>(a, vector<d>(b, c))
#define VVV(a, b, c, d) vector<vvl>(a, vvl(b, vll (c, d)));
#define re(c, b) for(ll c=0;c<b;c++)
#define rep(a,b,c) for(ll a=b;a<c;a++)
#define all(obj) (obj).begin(), (obj).end()
typedef long long int ll;
typedef long double ld;
using namespace std;

void get(vll &a){re(i,a.size()) scanf("%lld",&a[i]);}
void get(vvl &a){re(i,a.size()) re(j,a[0].size()) scanf("%lld",&a[i][j]);}
void print(vll &a){re(i,a.size()) cout<<a[i]<<(i==a.size()-1?"\n":" ");}
void print(vvl &a){re(i,a.size()) re(j,a[0].size())cout<<a[i][j]<<(j==a[0].size()-1?"\n":" ");}

int main(int argc, char const *argv[]) {
  ll n, m;std::cin >> n >> m;
  vll a(n);get(a);
  //区画　人数　和
  vvl d = VV(n, 0, 0, ll);

  re(i, m){
    ll x, w;scanf("%lld %lld", &x,&w);
    d[x-1].push_back(w);
  }
  ll al = 0;
  re(i, n){
    ll s = 0;
    for(auto e:d[i]) s += e;
    al += s;
    if(s>=a[i]){
      std::cout << -1 << '\n';
      return 0;
    }
  }
  bool g = true;
  re(i, n) if(a[i]<=al) g = false;
  if(g){
    std::cout << 0 << '\n';
    return 0;
  }


  //可能
  ll l = 0, r = 1000000000;
  while(r-l>1){
    ll mid = (l+r)/2;
    vvl q;//idx, クエリ , -1:= dowbn, -2消す
    //各要素について、影響を与える距離がわかる
    for(int i=0;i<n;i++){
      for(auto e:d[i]){
        ll x = e/mid;
        ll st;
        if(i<x) st = e - mid*i;
        else st = e%mid;
        q.push_back({max(i-x, 0LL), st, 0});
        q.push_back({i, -1, 0});
        q.push_back({i+x, -2, st});
      }
    }
    sort(all(q));
    ll idx = 0, s = 0, up = 0, down = 0;
    vll cost(n, 0);
    for(int i=0;i<n;i++){
      ll sa = mid*(up - down);
      s += sa;
      ll sub = 0;
      while(idx<q.size()&&q[idx][0]==i){
        if(q[idx][1]>=0) s += q[idx][1], up++;
        if(q[idx][1]==-1) up--, down++;
        if(q[idx][1]==-2) down--, sub += q[idx][2];
        idx++;
      }
      cost[i] = s;
      s -= sub;
    }
    bool f = true;
    for(int i=0;i<n;i++){
      if(cost[i]>=a[i]){
        f = false;
        break;
      }
    }
    if(f) r = mid;
    else l = mid;
  }//l no    r ok
  std::cout << r << '\n';
  return 0;
}