#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

ifstream cin ("orase.in");
ofstream cout ("orase.out");

int main()
{
	int m, n; cin >> m >> n;
	vector<pair<int, int>> v (n);

	for (int i = 0; i < n; i ++)
	{
		cin >> v[i].first >> v[i].second;
	}

	sort (v.begin(), v.end());
	int bst = v[0].first - v[0].second, ans = 0;

	for (int i = 1; i < n; i ++)
	{
		ans = max (ans, v[i].first + v[i].second - bst);
		bst = min (bst, v[i].first - v[i].second);
	}

	cout << ans << '\n';
	return 0;
}