#include<stdio.h>
#include<algorithm>
using namespace std; 
int n,i,p,M,m;
FILE *fin,*fout;
/*int fct(int a,int b){
return a>=b;
}
int main(){
	fin=fopen("orase.in","r");
	fout=fopen("orase.out","w");
	fscanf(fin,"%d",&n);
	for(i=1;i<=n;i++){
		fscanf(fin,"%d",&v[i]);
	}
	
	sort(v+1,v+n+1,fct);
	for(i=1;i<=n;i++){
		fprintf(fout,"%d",v[i]);
	}	
return 0;}
*/
struct orase{
int l,d;
}v[50001];
int fct(orase a, orase b){
return a.d<=b.d;
}
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",&v[i].d,&v[i].l);
	}
	sort(v+1,v+n+1,fct);
	p=1;
	for(i=2;i<=n;i++){
		if(v[i].d-v[p].d+v[i].l+v[p].l>M){
			M=v[i].d-v[p].d+v[i].l+v[p].l;
		}
		if(v[i].l-v[i].d>v[p].l-v[p].d){p=i;}
		
	}
	fprintf(fout,"%d",M);
	fclose(fin);
	fclose(fout);
return 0;}