// orase.cpp : Defines the entry point for the console application.
//

#include <stdio.h>
#include <cstdlib>

struct it
{
	int a,b;
}e[50001];
int n,m,dist=0;

int compar(const void *a ,const void *b)
{
	return(*(int*)a-*(int*)b);
}
int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	scanf("%d %d",&m,&n);
	for(int i=1;i<=n;i++)
		scanf("%d %d",&e[i].a,&e[i].b);
	qsort(e+1,n,sizeof(e[1]),compar);
	for(int i=1;i<=n;i++)
		for(int j=i+1;j<=n;j++)
			if(e[j].a-e[i].a+e[i].b+e[j].b>dist)
				dist=e[j].a-e[i].a+e[i].b+e[j].b;
	printf("%d",dist);

	return 0;
}