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

struct lala
{
	int x,y;
};

struct lala s[101000];

int co(const void *a,const void*b)
{
	if ((*(struct lala*)a).x==(*(struct lala*)b).x)
		return (*(struct lala*)a).y-(*(struct lala*)b).y;
	return ((*(struct lala*)a).x-(*(struct lala*)b).x);

}

int main()
{	
	int n,d,i,p;
	while (1)
	{
		scanf("%d",&n);
		if (n==0) break;
		for (i=0;i<n;i++) scanf("%d %d",&(s[i].x),&(s[i].y));
		d=0;	
		qsort(s,n,sizeof(s[0]),co);
		for (i=0;i<n;i+=2) d+=s[i+1].y-s[i].y;
		for (i=0;i<n;i++) 
		{
			p=s[i].x;
			s[i].x=s[i].y;
			s[i].y=p;
		}
		qsort(s,n,sizeof(s[0]),co);
		for (i=0;i<n;i+=2) d+=s[i+1].y-s[i].y;
		printf("The length of the fence will be %d units.\n",d);
	}
	return 0;
}