#include <iostream>
#include <fstream>
using namespace std;
int D[50000];
int L[50000];


int main()
{
	ifstream citire("orase.in");
	ofstream scriere("orase.out");
	int M,N,i,j,Max_curent,Max;
	citire>>M>>N;
	Max=-1;
	for(i=0;i<N;i++)
	{
		citire>>D[i]>>L[i];
	}
	for(i=0;i<N-1;i++)
	{
		for(j=i+1;j<N;j++)
		{
			
			
			if(D[i]>D[j])
			{
				Max_curent=L[i]+L[j]+D[i]-D[j];
			}
			else if(D[j]>D[i])
			{
				Max_curent=L[i]+L[j]+D[j]-D[i];
			}
			if(Max_curent>Max)
			{
				Max=Max_curent;
			}
			
		}
	}
	scriere<<Max;
}