#include<stdio.h>
#include<algorithm>
#define NMAX 50051
using namespace std;
long n,m,a,i,s,mx,k,l,q;
struct kkt
{
	long X,Y;
};
kkt x[NMAX];
int cmpf(const kkt a,const kkt b)
{
	return a.X<b.X;
}
int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	scanf("%ld%ld",&m,&n);
	for (i=1;i<=n;i++)
		scanf("%ld%ld",&x[i].X,&x[i].Y);
/*	a=1;
	while (a)
	{
		a=0;
		for (i=1;i<n;i++)
		if (x[i].X>x[i+1].X)
		{
			a=x[i].X;
			x[i].X=x[i+1].X;
			x[i+1].X=a;
			a=x[i].Y;
			x[i].Y=x[i+1].Y;
			x[i+1].Y=a;
			a=1;
		}
	} */
	sort(x+1,x+n+1,cmpf);
	s=/*x[1].X+*/x[1].Y;
	mx=-1;
	for (i=2;i<=n;i++)
	{
		if (mx<s+x[i].X-x[i-1].X+x[i].Y)
			mx=s+x[i].X-x[i-1].X+x[i].Y;
		if (s+x[i].X-x[i-1].X<=x[i].Y)
			s=x[i].Y;
		else
		s+=(x[i].X-x[i-1].X);
	}
	printf("%ld\n",mx);


	return 0;
}