#include <stdio.h>
int m,n,d[50001],l[50001],i,j,max;
int main() {
	FILE *f=fopen("orase.in","r");
	FILE *g=fopen("orase.out","w");
	fscanf(f,"%d%d",&m,&n);
	for(i=1;i<=n;i++)
		fscanf(f,"%d%d",&d[i],&l[i]);
	for(i=1;i<=n;i++)
		for(j=1;j<=n;j++)
			if(d[i]-d[j]>0) {
				if(l[i]+l[j]+d[i]-d[j]>max)
					max=l[i]+l[j]+d[i]-d[j];
			}
			else
				if(l[i]+l[j]-d[i]+d[j]>max)
					max=l[i]+l[j]-d[i]+d[j];
	fprintf(g,"%d",max);
	return 0;
}