#include <stdio.h>
long m,n,ml1,ml2,x,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=d1-d2;
			if (md<0)md*=-1;
		}
		else
		if(ml2<l){
			ml2=l;
			d2=d;
			md=d1-d2;
			if (md<0)md*=-1;
		}
		else
		if(ml1==l){
			x=d-d2;
			if (x<0) x*=-1;
			if(x>md){
				d1=d;
				md=d1-d2;
				if (md<0)md*=-1;
			}
		}
		else
		if(ml2==l){
			x=d1-d;
			if (x<0) x*=-1;
			if(x>md){
				d2=d;
				md=d1-d2;
				if (md<0)md*=-1;
			}
		}
	}
	fprintf(g,"%ld\n",ml1+ml2+md);
	fclose(f);
	fclose(g);
	return 0;
}