#include<stdio.h>
#include<math.h>
FILE *fin,*fout;
int m,i,n,d[50000],l[50000],x[3],dist,max=0,j;
int main(){
	fin=fopen("orase.in","r");
	fout=fopen("orase.out","w");
	fscanf(fin,"%d %d",&m,&n);
	for(i=1;i<=n;i++){
		fscanf(fin,"%d %d",&d[i],&l[i]);
	}
	x[0]=0;
	for(i=1;i<=n;i++){
		for(j=1;j<i;j++){
			dist=d[i]-d[j];
			if(dist+l[i]+l[j]>max){max=dist+l[i]+l[j];}
			if(dist*-1+l[i]+l[j]>max){max=dist*-1+l[i]+l[j];}
		}
	}
	fprintf(fout,"%d",max);
	fclose(fin);
	fclose(fout);
return 0;}