#include <iostream>
#include <fstream>
using namespace std;



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