#include<fstream>
#include<algorithm>
#include<limits.h>
#define dim 50007
using namespace std;


ifstream f("orase.in");
ofstream g("orase.out");
int n,Steaua,Becali,antrenor,Dinamo,i,m;
struct cub {
	int x,y;
};
cub A[dim];
bool cmp (cub a, cub b){
	
	return a.x<b.x;
	
}
int main (){
	f>>m>>n;
	
	Becali=INT_MAX;
	
	for(i=1;i<=n;i++){
		
		f>>A[i].x>>A[i].y;
		
	}
	
	sort(A+1,A+1+n,cmp);
	
	Steaua=-Becali;
	antrenor=1;
	
	for(i=2;i<=n;++i){
		
		Dinamo=A[i].x-A[antrenor].x+A[i].y+A[antrenor].y;
		if(Steaua<Dinamo)
			Steaua=Dinamo;
		
		if(A[i].y>A[i].x-A[antrenor].x+A[antrenor].y)
			antrenor=i;
		
	}
	g<<Steaua<<"\n";
	return 0;
}