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

int n;

struct bod {
  int x;
  int y;
} T[110000];

int cmpx(const void *a, const void *b) {
  struct bod *A, *B;
  A=a; B=b;
  if (A->x<B->x) return -1;
  if (A->x>B->x) return 1;
  return 0;
}

int cmpy(const void *a, const void *b) {
  struct bod *A, *B;
  A=a; B=b;
  if (A->y<B->y) return -1;
  if (A->y>B->y) return 1;
  return 0;
}

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