#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
long N,M,sum,max1;
struct dist { int d,l; }dp[50002];
bool cmp(dist i,dist j) {	return (i.d<j.d); }
int main()
{	f>>M>>N;
	for(int i=1;i<=N;i++)
		f>>dp[i].d>>dp[i].l;
	sort(dp+1,dp+N+1,cmp);
	max1=dp[1].l-dp[1].d;
	for(int i=2;i<=N;i++)
	{	if(sum<dp[i].l+dp[i].d+max1)
			sum=dp[i].l+dp[i].d+max1;
		if(max1<dp[i].l-dp[i].d)
			max1=dp[i].l-dp[i].d;
	}
	g<<sum;
	f.close();
	g.close();
	return 0;
}