#include <fstream.h>
#include <algorithm>
#define M 1000001
#define N 50001

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

int n,m,best = -int(2e9), maxj = 0; // ds[N] distante strazi , dso[N] distanta strazi orase

struct distance{
	int ds;
	int dso;
} d[N];

int cmp(const distance &x,const distance &y){
	return (x.ds < y.ds);
}

void read(){
	int i;
	
	fin>>m>>n;
	
	for (i=1;i<=n;i++)
		fin>>d[i].ds>>d[i].dso;
}

void solve(){
	int i;
	
	maxj = d[1].dso-d[i].ds;
	for (i=2; i<=n; i++){
		if ( d[i].ds + d[i].dso + maxj > best)
			best = d[i].ds + d[i].dso + maxj;
		
		if ( d[1].dso-d[i].ds > maxj )
			maxj = d[1].dso-d[i].ds;
	}	
}

void qsort(int s, int d){
	int i=s,j=d,pi=0,pj=1,aux;
	
	if (s<d){
	
		while(i<j){
			if (ds[i] > ds[j]){
				aux = ds[i];
				ds[i] = ds[j];
				ds[j] = aux;
				
				aux = dso[i];
				dso[i] = dso[j];
				dso[j] = aux;
				
				aux = pi;
				pi = pj;
				pj = aux;
			}
			i += pi; j -= pj;
		}
		
		qsort(s,i-1);
		qsort(m+1,d);
	
	}
}

int main(){

	read();
	//qsort(1,n);
	sort(d+1,d+n+1,cmp);
	solve();
	
	//for (int i=1;i<=n;i++)
	//	fout<<ds[i]<<","<<dso[i]<<" ";
	fout<<best;
	return 0;
}