#include<stdio.h>
long int d[50000],l[50000],n,m,i,dist,j;
int main()
{   FILE *f=fopen("orase.in","r");
    FILE *g=fopen("orase.out","w");
    fscanf(f,"%ld%ld",&m,&n);
    for(i=1;i<=n;i++){  fscanf(f,"%ld%ld",&d[i],&l[i]);
                        if(i>1)
                         {if(i==2) if(d[2]>=d[1]) dist=l[1]+l[2]+(d[2]-d[1]);
                                       else dist=l[1]+l[2]+(d[1]-d[2]);
                           else for(j=i-1;j>=1;j--)if(d[i]<=d[j]) if(dist<(l[j]+l[i]+(d[i]-d[j]))) dist=l[j]+l[i]+(d[i]-d[j]);
                                                      else  if(dist<(l[j]+l[i]+(d[j]-d[i]))) dist=l[j]+l[i]+(d[j]-d[i]);
                           }
                        }
    fprintf(g,"%ld",dist);
    fcloseall();
    return 0;
}