#include <stdio.h>
#include <math.h>
long m,n,ml1,ml2,md,d1,d2,d,l,i;
int main(){
	FILE *f,*g;
	f=fopen("orase.in","r");
	g=fopen("orase.out","w");
	fscanf(f,"%ld%ld",&m,&n);
	for(i=1;i<=n;i++){
		fscanf(f,"%ld%ld",d,l);
		if(ml1<l){
			ml2=ml1;
			ml1=l;
			d2=d1;
			d1=d;
			md=labs(d1-d2);
		}
		else
		if(ml2<l){
			ml2=l;
			d2=d;
			md=labs(d1-d2);
		}
		else
		if(ml1==l)
			if(labs(d-d2)>md){
				d1=d;
				md=labs(d1-d2);
			}
			else;
		else
		if(ml2==l)
			if(labs(d1-d)>md){
				d2=d;
				md=labs(d1-d2);
			}
	}
	fprintf(g,"%ld\n",ml1+ml2+md);
	fclose(f);
	fclose(g);
	return 0;
}