n=int(input())
a=[tuple(map(int,input().split())) for i in range(n)]
p={}
for i in range(n):
  ax,ay=a[i]
  dx1,dy1=2,0
  dx2,dy2=0,1
  for j in range(4):
    bx,by=ax+dx1+dx2,ay+dy1+dy2
    b=(bx,by)
    if b not in a:
      if b not in p:
        p[b]=[0]*4
      p[b][j-2]|=1<<i
    bx,by=ax+dx1-dx2,ay+dy1-dy2
    b=(bx,by)
    if b not in a:
      if b not in p:
        p[b]=[0]*4
      p[b][j-2]|=1<<i
    dx1,dy1=-dy1,dx1
    dx2,dy2=-dy2,dx2
X=100
q=[X]*(1<<n)
q[0]=0
for b in p:
  for i in reversed(range(1<<n)):
    for m in p[b]:
      q[i|m]=min(q[i|m],q[i]+1)
print(q[-1] if q[-1]<X else -1)