#include<fstream>
#include<algorithm>
using namespace std;
ifstream fi("orase.in");
ofstream fo("orase.out");
typedef struct coord{int x,y;} COORD;
COORD A[50001];
int m,n,i,a,dist,maxim;

bool cmp(COORD a, COORD b)
{
	return a.x<b.x;
}

int main()
{
	fi>>m>>n;
	for(i=1; i<=n; i++)
		fi>>A[i].x>>A[i].y;
	sort(A+1,A+n+1,cmp);
	a=A[1].y-A[1].x;
	for(i=2; i<=n; i++)
	{
		dist=A[i].x+A[i].y+a;
		if(dist>maxim)
			maxim=dist;
		if(A[i].y-A[i].x>a)
			a=A[i].y-A[i].x;
	}
	fo<<maxim<<"\n";
	fi.close();
	fo.close();
	return 0;
}