// orase.cpp - infoarena.ro

#include <fstream>
#include <algorithm>

using namespace std;

pair <int, int> s[50000];

int main()
{
	int n;
	ifstream in("orase.in");
	in >> n >> n;
	for(int i = 0; i < n; ++i)
		in >> s[i].first >> s[i].second;	
	in.close();

	sort (s, s+n);

	int x = s[0].first, y = s[0].second, c, d, S;

	for (int i = 1; i < n; ++i)
	{
		c = s[i].first;
		d = s[i].second;
		S = max(S, c - x + y + d);
		y = max(y + c - x, d);
		x = c;
	}
	
	ofstream out("orase.out");
	out << S;
	out.close();
	
	return 0;
}