#include <stdio.h>

int n;

struct co {int x[2];};

struct co cs[200000];
struct co coss[200000];
struct co coss2[200000];
int starts[200000];

void bucket(int s, struct co* from, struct co *to)
{
	int i;

	memset(starts, 0, 10002 * sizeof (int));

	for (i = 0; i < n; i++)
		starts[from[i].x[s]+1]++;
	for (i = 0; i <= 10000; i++)
		starts[i+1]+= starts[i];

	for (i = 0; i < n; i++)
		to[starts[from[i].x[s]]++] = from[i];
}

int main(void)
{
	int i;
	unsigned long long sum;

	while (1)
	{
		scanf("%d", &n);

		if (!n)
			return 0;

		for (i = 0; i < n; i++)
			scanf("%d%d", cs[i].x, cs[i].x + 1);

		sum = 0;

		bucket(0, cs, coss);
		bucket(1, coss, coss2);
		
		for (i = 0; i < n; i+=2)
		{
			sum += coss2[i+1].x[0] - coss2[i].x[0];
		}
		bucket(1, cs, coss);
		bucket(0, coss, coss2);
		for (i = 0; i < n; i+=2)
		{
			sum += coss2[i+1].x[1] - coss2[i].x[1];
		}

		printf("The length of the fence will be %lld units.\n", sum);
	}
	
	return 0;
}