#include<iostream>
#include<vector>
using namespace std;

int mat[403][403][403];
//mat[current_location][final_location][refuel_count_left]
vector<int> city;
int fun(int s,int f, int r){
    if(s==f){
        return mat[s][f][r]  = 0;
    }
    if(mat[s][f][r]!=0){
        return mat[s][f][r];
    }
    if(s+1==f || r==0){
        return mat[s][f][r] = city[f]-city[s];
    }

    int opt = city[s]  + (city[f]-city[s])/(r+1);


    int pos  = lower_bound(city.begin()+s,city.begin()+f+1,opt) - city.begin();

    mat[s][f][r] = max(fun(pos,f,r-1), city[pos] - city[s]);

    //cout<<"s=" <<s<<" f="<<f<<" r="<<r<<" mat"<<mat[s][f][r]<<endl;

    return mat[s][f][r];
}
int main(){

    long long ans=0;
    int no_cities,no_vehicle;
    cin>>no_cities>>no_vehicle;

    city.push_back(0);
    for(int i=1;i<=no_cities;++i){
        int b;cin>>b;
        city.push_back(b);
    }
    for(int i=0;i<no_vehicle;++i){
        int start,finish,costperkm,refuel_count;
        cin>>start>>finish>>costperkm>>refuel_count;
        long long  vol_curr_vehicle = (long long)fun(start,finish,refuel_count)* (long long)costperkm;
        //cout<<jj<<" ";
        ans = max(ans,vol_curr_vehicle);
    }
    printf("%lld",ans);
    return 0;
}