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


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;
	for(i=1;i<=n;++i)
		for(j=1;j<=n;++j)
			if(i!=j)
			{
				s=a[j].d-a[i].d+a[j].l+a[i].l;
				if(s>max)
					max=s;
			}
	out<<max<<"\n";
	return 0;
}