#include<stdio.h>

long absv(long n)
{
    return n<0 ? -n : n;
}

long calcdist(long a,long b, long c,long d)
{
    if(a==c)
    return absv(b-d);
    else
    return b+d+absv(a-c);
}

int main()
{
    long m;
    long d[50001],l[50001];
    int imax=0,i,n;
    long max=0;
    FILE *fin,*fout;
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");
    fscanf(fin,"%ld%d",&m,&n);
    for(i=1;i<=n;i++)
    {
        fscanf(fin,"%ld%ld",&d[i],&l[i]);
        for(int j=1;j<i;j++)
        {
            long ret=calcdist(d[i],l[i],d[j],l[j]);
            if(!imax){imax=1; max=ret;}
            else if(max<ret) max=ret; 
        }
    }
    fprintf(fout,"%ld",max);
    fclose(fin);
    fclose(fout);
    return 0;
}