#include<cstdio>
#include<vector>
#include<algorithm>

using namespace std;

vector<pair<int,int> > v;
int m,n;

void citire()
{
	int x,y;
	scanf("%d%d",&m,&n);
	for(int i=0;i<n;++i)
	{
		scanf("%d%d",&x,&y);
		v.push_back(make_pair(x,y));
	}
}

int calcul()
{
	int rez=0,dmax=v[1].second,i;
	for(i=1;i<n;++i)
	{
		rez=max(rez , dmax + v[i].second + v[i].first-v[i-1].first);
		dmax=max(dmax + v[i].first-v[i-1].first , v[i].second);
	}
	return rez;
}

int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);
	citire();
	sort(v.begin(),v.end());
	printf("%d\n",calcul());
	return 0;
}