#include<bits/stdc++.h>
#define ALL(v) std::begin(v),std::end(v)
using lint=long long;
using ld=long double;
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    std::cout.setf(std::ios_base::fixed);std::cout.precision(15);
    lint n,m;std::cin>>n>>m;
    std::vector<lint>a(n);
    for(lint&x:a)std::cin>>x;
    std::vector<std::pair<lint,lint>>xw(m);
    for(auto&&[x,w]:xw){
        std::cin>>x>>w;x--;
    }
    auto ok=[&](lint c){
        if(c==0){
            lint sum=0;
            for(auto&&[x,w]:xw)sum+=w;
            return std::all_of(ALL(a),[sum](lint x){return sum<=x;});
        }
        std::vector<lint>b(n+1);
        for(auto&&[x,w]:xw){
            lint q=w/c;
            b.at(std::max(0ll,x-q)+1)+=c;
            b.at(x+1)-=2*c;
            if(x+q+1<=n)b.at(x+q+1)+=c;
        }
        std::partial_sum(ALL(b),b.begin());
        for(auto&&[x,w]:xw){
            lint q=w/c,r=w%c;
            b.at(std::max(0ll,x-q))+=r+c*std::max(0ll,q-x);
            if(x+q+1<=n)b.at(x+q+1)-=r;
        }
        std::partial_sum(ALL(b),b.begin());
        for(lint i=0;i<n;i++){
            if(a.at(i)<b.at(i))return false;
        }
        return true;
    };
    lint inf=std::numeric_limits<lint>::max()/3;
    lint l=-1,r=inf;
    while(1<r-l){
        lint c=(l+r)/2;
        (ok(c)?r:l)=c;
    }
    std::cout<<(r==inf?-1:r)<<'\n';
}