#include <fstream>
#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
using namespace std;
//ifstream f("orase.in");
//ofstream g("orase.out");
FILE *f=fopen("orase.in","r"),*g=fopen("orase.out","w");
struct orase{
	int li,di;
}v[1000001];
int cmp( orase a, orase b){
	if(a.di<b.di)
		return 1;

		return 0;
}
	
int n,m,i;
long long bun;
int main()
{
	long long max=0;;
	fscanf(f,"%d %d",&m,&n);
	for(i=1;i<=n;i++)
		fscanf(f,"%d %d",&v[i].di,&v[i].li);
	sort(v+1,v+n+1,cmp);
	bun=v[1].li-v[2].di;
	for(i=2;i<=n;i++){
		if(v[i].li+v[i].di+bun>max)
			max=v[i].li+v[i].di+bun;
		if(v[i].li-v[i].di>bun)
			bun=v[i].li-v[i].di;
	}
	fprintf(g,"%lld",max);
return 0;
}