#include <cstdio>
#include <algorithm>

using namespace std;

#define MAX_N 50005
#define INF 0x3f3f3f

#define d first
#define l second

int N, M;
pair<int, int> A[MAX_N];

void citire()
{
	scanf("%d %d",&M, &N);
	
	for(int i = 1; i <= N; ++i)
	{
		int a, b;
		scanf("%d %d",&a, &b);
		A[i] = make_pair(a, b);
	}
	sort(A+1, A+N+1);
}

void solve()
{
	int m(-INF), sol(-INF);
	
	for(int i = 1; i <= N; ++i)
	{
		sol = max(sol, A[i].d + A[i].l + m);
		m = max(m, A[i].l - A[i].d);
	}
	
	printf("%d\n",sol);
}

int main()
{
	freopen("orase.in","rt",stdin);
	freopen("orase.out","wt",stdout);
	
	citire();
	solve();
}