#include <bits/stdc++.h>
using namespace std;
#define modulo 1000000007
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 1000000000

int main(){

	int N,M;
	cin>>N>>M;
	
	vector<long long> a(N);
	for(int i=0;i<N;i++){
		scanf("%lld",&a[i]);
	}
	
	vector<pair<long long,long long>> V(M);
	for(int i=0;i<M;i++){
		scanf("%lld %lld",&V[i].first,&V[i].second);
		V[i].first--;
	}
	
	long long ok = 100005;
	long long ng = -1;
	
	while(ok-ng>1){
		long long c = (ok+ng)/2;

		vector<vector<int>> add(N,vector<int>()),del(N,vector<int>());

		for(int i=0;i<M;i++){
			if(c==0){
				add[0].push_back(i);
				continue;
			}
			long long x = V[i].second/c;
			add[max(V[i].first-x,0LL)].push_back(i);
			del[min(V[i].first+x,(long long)N-1)].push_back(i);
		}
		
		bool f = true;
		
		long long sum = 0;
		multiset<int> M1,M2;

		
		for(int i=0;i<N;i++){
			sum -= c*M1.size();
			sum += c*M2.size();
			
			int C = M2.count(i);
			M2.erase(i);
			for(int j=0;j<C;j++)M1.insert(i);
			for(int j=0;j<add[i].size();j++){
				int ind = add[i][j];
				if(i>=V[ind].first){
					M1.insert(ind);
				}
				else{
					M2.insert(ind);
				}
				sum += V[ind].second - abs(i-V[ind].first)*c;
			}

			if(a[i]<=sum){
				f=false;
				break;
			}
			
			for(int j=0;j<del[i].size();j++){
				int ind = del[i][j];
				if(M1.count(ind)){
					M1.erase(M1.lower_bound(ind));
					sum -= V[ind].second - abs(i-V[ind].first)*c;
				}
				else{
					M2.erase(M2.lower_bound(ind));
					sum -= V[ind].second - abs(i-V[ind].first)*c;
				}
			}
		}

		if(f)ok = c;
		else ng = c;
		
	}
	
	if(ok==100005)ok=-1;
	cout<<ok<<endl;
	
    return 0;
}