#include<stdio.h>
#include<stdlib.h>
struct muie{
	int d,l;
};
int comp(const void *a,const void *b){
	int *aa=(int *)a, *bb=(int *)b;
	int x=*aa, y=*bb;
	return x-y;
}
int main () {
	int m,n,i,j,max=-1,a;
	muie v[50000];
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	scanf("%d%d",&m,&n);
	for(i=0;i<n;++i)
		scanf("%d%d",&v[i].d,&v[i].l);
	qsort(v,n,sizeof(v[0]),comp);
	for(i=0;i<n-1;++i)
		for(j=i+1;j<n;++j){
			a=v[i].l-v[i].d+v[j].d+v[j].l;
			
			if(a>max)
				max=a;
		}
	printf("%d\n",max);
	return 0;
}