#include<fstream>
#include<algorithm>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
long long i,m,n,mpoz,maxim,j;
pair<int,int>p[50001];
int main(){
    fin>>m>>n;
    for(i=1;i<=n;i++){
        fin>>p[i].first>>p[i].second;
    }
    sort(p+1,p+n+1);
    j=1;
    mpoz=p[1].second-p[1].first;
    for(i=2;i<=n;i++){
        if(maxim<p[j].second+p[i].second+(p[i].first-p[j].first)){
            maxim=p[j].second+p[i].second+(p[i].first-p[j].first);
        }
        if(mpoz<p[i].second-p[i].first){
            mpoz=p[i].second-p[i].first;
            j=i;
        }
    }
    fout<<maxim;
    return 0;
}