#include <stdio.h>
int poz[50000],l[50000];

int main()
{
    FILE *in,*out;
    int m,n,i,j,max,maxm=0;
    in=fopen("orase.in","r");
    out=fopen("orase.out","w");
    fscanf(in,"%d%d",&m,&n);
    for(i=0;i<n;i++)
    {
        fscanf(in,"%d%d",&poz[i],&l[i]);
    }
    for(i=0;i<n;i++)
    {
        for(j=i+1;j<n;j++)
        {
            max=0;
            if(poz[i]<poz[j])
                max+=poz[j]-poz[i];
            else
                max+=poz[i]-poz[j];
            max+=l[i]+l[j];
            if(max>maxm)
                maxm=max;
        }
    }
    fprintf(out,"%d",maxm);
    return 0;
}