#include<stdio.h>
#include<stdlib.h>
int d[1000000],l[1000000];
int main()
{
	FILE *in,*out;
	in = fopen("orase.in","r");
	out = fopen("orase.out","w");
	int M,N,max = -1,i,j,x;
	fscanf(in,"%d", &M);
	fscanf(in,"%d", &N);
	for(i = 1;i <= N;i++)
	{
		fscanf(in,"%d", &d[i]);
		fscanf(in,"%d", &l[i]);
	}
	for(i = 1;i <= N;i++)
	{
		for(j = i+1;j <= N;j++)
		{
			x = l[i]+l[j]+abs(d[i]-d[j]);
			if(max < x)
				max = x;
		}
	}
	fprintf(out,"%d\n", max);
	return 0;
}