#include <iostream>
#include <cstdio>

using namespace std;
int abs(int k,int l)
{
    int dif=k-l;
    if(dif<0)
        dif=0-dif;
    return dif;
}
int strazil[100001],strazip[100001];
int sol(int r1,int r2){
   int r=abs(strazip[r1],strazip[r2])+strazil[r1]+strazil[r2];
   return r;
};
int main()
{
    /*freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);*/
    int n,c,i,r1,r=0,j;
    scanf("%d%d", &n,&c);
    for(i=1;i<=c;i++)
    {
        scanf("%d%d", &strazip[i],&strazil[i]);
    }
    for(i=1;i<=n-1;i++)
    {
        for(j=i+1;j<=n;j++)
        {
            r1=sol(i,j);
            if(r1>r)
                r=r1;
        }
    }
    printf("%d", r);
    return 0;
}