#include <fstream>
#include <algorithm>

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

const int nmax=50000;

struct str{
    int d,l;
}v[nmax+1];

struct cmp{
    bool operator()(str x, str y){
        return x.d<y.d;
    }
};

int main(){
    int m,n;
    fin>>m>>n;
    for(int i=1;i<=n;i++){
        fin>>v[i].d>>v[i].l;
    }
    sort(v,v+n+1,cmp());
    int sol=0,p=1;
    for(int i=2;i<=n;i++){
        if(v[i].l-v[i].d>v[p].l-v[p].d){
            p=i;
        }
        int sum=v[i].l+v[p].l+v[i].d-v[p].d;
        if(sum>sol){
            sol=sum;
        }
    }
    fout<<sol<<"\n";
    return 0;
}