#include <bits/stdc++.h>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int m, n, maxi;
struct dimi{
    int x, y;
}v[50002];
inline bool cmp(const dimi a, const dimi b)
{
    if(a.x==b.x)
        return a.y<b.y;
    return a.x<b.x;
}
int main()
{
    int i;
    fin>>m>>n;
    for(i=1; i<=n; i++)
        fin>>v[i].x>>v[i].y;
    sort(v+1,v+n+1,cmp);
    for(i=1; i<=n-1; i++)
        maxi=max(maxi,(-1)*v[i].x+v[i].y+v[n].x+v[n].y);
    fout<<maxi<<'\n';
    return 0;
}