#include <fstream>
#include <vector>
#include <algorithm>
#define l first
#define d second
using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

typedef pair <int, int> p;

int n, m;

p a;

vector <p> v;

int main()
{
	fin >> m >> n;
	
	for (int i = 1; i <= n; ++i)
	{
		fin >> a.l >> a.d;
		v.push_back(a);
	}

	sort(v.begin(), v.end());

	int max_dist = v[0].d, poz = 0;

	for (size_t i = 1; i < n; ++i)
	{
		if (v[i].d + v[poz].d + v[i].l - v[poz].l > max_dist)
			max_dist = v[i].d + v[poz].d + v[i].l - v[poz].l;
		if (v[i].second > v[i].l - v[poz].l + v[poz].d)
			poz = i;
	}
	fout << max_dist << "\n";
	fout.close();
	return 0;
}