#include<stdio.h>
#include<algorithm>
using namespace std;
FILE*f=fopen("orase.in","r");
FILE*g=fopen("orase.out","w");
int n,m;

struct oras
{
	int x,y;
} v[50001];
int cmp (oras a,oras b)
{
	return a.x<b.x;
}
int main()
{
	fscanf(f,"%d%d",&m,&n);
	for(int i=1;i<=n;++i)
		fscanf(f,"%d%d",&v[i].x,&v[i].y);
	
	sort(v+1,v+n+1,cmp);
	
	int d=v[1].y-v[1].x;
	int maxd=0;
	for(int i=2;i<=n;++i)
	{
		if(d+v[i].x+v[i].y>maxd)
			maxd=d+v[i].x+v[i].y;
		if(d<v[i].y-v[i].x)
			d=v[i].y-v[i].x;
		
	}
	
	fprintf(g,"%d",maxd);
	fclose(f);
	fclose(g);
	return 0;
}