#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin("orase.in");
ofstream cout("orase.out");

struct oras
{
    int x,y;
};
oras v[50002];

bool cmp(oras a,oras b)
{
    return a.x<b.x;
}

int main()
{
    int m,n,i,max=-100,j;
    cin>>m;
    cin>>n;
    for(i=1; i<=n; i++)
        cin>>v[i].x>>v[i].y;
    sort(v+1,v+n+1,cmp);
    for(i=1; i<n; i++)
        for(j=i+1;j<=n;j++)
            if(v[j].x-v[i].x+v[j].y+v[i].y>max)
                max=v[j].x-v[i].x+v[j].y+v[i].y;
    cout<<max;
    cin.close();
    cout.close();
    return 0;
}