#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

int get_line(char *s, int max)
{
	int i;
	fgets(s,max,stdin);
	i = strlen(s) - 1;
	while (i >= 0 && s[i] == '\n')
		i--;
	i++;
	s[i] = 0;
	return i;
}

int sgetint(char **s, int *x)
{
	char *t = *s;
	
	while (isspace(*t)) t++;
	if (sscanf(t,"%d",x) != 1) return 0;
	while (*t && !isspace(*t)) t++;
	*s = t;
	return 1;
}

struct point {
	int coord;
	struct point * next;
};

struct point pts[200010];
int nextpt;
struct point * xx[10002];
struct point * yy[10002];
int pole[10002];

int vysyp(struct point * list, int * pole)
{
	int * q = pole;
	while (list) {
		*q++ = list->coord;
		list = list->next;
	}
	return q - pole;
}
int komparator(int *a, int *b)
{
	return (*a<*b) ? -1 : (*a > *b);
}
int projdi(int * a, int i) /* a[i] je citelne */
{
	int total =0;
	int run = *a++;
	i >>= 1;
	while (i--) {
		total += *a++ - run;
		run = *a++;
	}
	return total;
}
/*
int lowcoord(struct point * list, int hranice)
{
	register int last = -1;
	register int c;
	while (list) {
		c = list->coord;
		if ((c < hranice) && (c > last)) {
			last = c;
		}
		list = list->next;
	}
	return last;
}
int highcoord(struct point * list, int hranice)
{
	register int last = 20000;
	register int c;
	register struct point * pp = NULL;
	while (list) {
		c = list->coord;
		if ((c > hranice) && (c < last)) {
			last = c;
			pp = list;
		}
		list = list->next;
	}
	if (pp) {
		if (pp->next) {
			pp->coord = pp->next->coord;
			pp->next = pp->next->next;
		} else {
			pp->coord = -10;
		}
	}
	return last;
} */

int main(void)
{
	int P,i,x,y,total;
	struct point * q;
	for (;;) {
		scanf("%d\n", &P);
		if (!P)
			break;
		memset(xx,0,sizeof(xx));
		memset(yy,0,sizeof(yy));
		nextpt = 0;
		for (i=0 ; i < P ; i++) {
			scanf("%d %d\n",&x,&y);
			q = xx[x];
			pts[nextpt].next = q;
			pts[nextpt].coord = y;
			xx[x] = pts + nextpt++;
			q = yy[y];
			pts[nextpt].next = q;
			pts[nextpt].coord = x;
			yy[y] = pts + nextpt++;
		}

		total = 0;
		for (x = 0 ; x < 10002 ; x++) {
			if (xx[x]) {
				i = vysyp(xx[x],pole);
				qsort(pole, i, sizeof(pole[0]), komparator);
				total += projdi(pole, i);
			}
		}
		for (x = 0 ; x < 10002 ; x++) {
			if (yy[x]) {
				i = vysyp(yy[x],pole);
				qsort(pole, i, sizeof(pole[0]), komparator);
				total += projdi(pole, i);
			}
		}
		printf("The length of the fence will be %d units.\n", total);

	}
	return 0;
}