#include<fstream>
#include<cstdlib>
using namespace std;
struct oras
{
	long d;
	long l;
};
oras a[50001];
ifstream in("orase.in");
ofstream out("orase.out");



int comp(const void *a,const void *b)
{
	oras *pa,*pb;
	pa=(oras*)a;
	pb=(oras*)b;
	return pa->d-pb->d;
}




int main()
{
	long max=0,s=0,i,j,n,m;
	in>>m>>n;
	for(i=1;i<=n;++i)
		in>>a[i].d>>a[i].l;
	qsort(a+1,n,sizeof(a[0]),comp);
	for(i=1;i<n;++i)
		for(j=i+1;j<=n;++j)
			{
				s=a[j].d-a[i].d+a[j].l+a[i].l;
				if(s>max)
					max=s;
			}
	out<<max<<"\n";
	return 0;
}