#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
struct oras
{
    int d,l;
};
int cmp(oras x,oras y)
{
    return x.d<y.d;
}
int main()
{
    oras o[50001];
    int n,m,i,j,l,lmax=0;
    in >> m >> n;
    for(i=1;i<=n;i++)
        in>>o[i].d>>o[i].l;
    sort(&o[1], &o[n+1],cmp);
    for(i=1;i<n;i++)
        for(j=i;j<=n;j++){
            l = o[i].l + (o[j].d - o[i].d) + o[j].l;
            if(l > lmax) lmax = l;
        }
    cout<<lmax;
    return 0;
}