#include<fstream>
#include<algorithm>
using namespace std;
struct oras
{
	int D,L;
} CITY[50001];
ifstream fi("orase.in");
ofstream fo("orase.out");

int cmp(oras A, oras B)
// returneaza 1 atunci cand A<B
{
	if (A.D<B.D)
		return 1;
	return 0;
}

int main()
{
	int m,n,i,maxd=0,j;
	fi>>m>>n;
	for(i=1;i<=n;i++)
		fi>>CITY[i].D>>CITY[i].L;
	sort(CITY+1,CITY+n+1,cmp);
	j=1;
	for (i=2;i<=n;i++)
	{
		if(CITY[i].D+CITY[i].L-CITY[j].D+CITY[j].L>maxd)
			maxd=CITY[i].D+CITY[i].L-CITY[j].D+CITY[j].L;
		if (CITY[i].L-CITY[i].D>CITY[j].L-CITY[j].D)
			j=i;
	}
    fo<<maxd;
	fi.close();
	fo.close();
	return 0;
}