#include<bits/stdc++.h>using namespace std;const int INF = 0x7FFFFFFF;const int mod = 1e9 + 7;const int N = 5e6 + 10;const int M = 1e4 + 1;const double eps = 1e-10;int T,n,m;struct P{    int x,y;    P(int X=0,int Y=0)    {        x=X;        y=Y;    }}p[2000],q[2000];bool cmp1(P a,P b){    if(a.x!=b.x) return a.x<b.x;    return a.y<b.y;}bool cmp2(P a,P b){    if(a.y!=b.y) return a.y<b.y;    return a.x<b.x;}int nowx,nowy,nowd;int main(){    while(~scanf("%d",&n))    {        for(int i=1;i<=n;i++)        {            scanf("%d%d",&p[i].x,&p[i].y);            q[i].x=p[i].x;            q[i].y=p[i].y;        }        sort(p+1,p+1+n,cmp1);        sort(q+1,q+1+n,cmp2);        nowx=nowy=0; nowd=0;        int ans=0;        while(1)        {            if(nowd==0)            {                int k = lower_bound(q+1,q+n+1,P(nowx,nowy),cmp2)-q;                if(k==n+1||q[k].y!=nowy)                {                    printf("%d\n",ans);                    break;                }                else ans++,nowx = q[k].x-1,nowd=(nowd+1)%4;            }            else if(nowd==2)            {                int k = lower_bound(q+1,q+n+1,P(nowx,nowy),cmp2)-q-1;                if(k==0||q[k].y!=nowy)                {                    printf("%d\n",ans);                    break;                }                else ans++,nowx = q[k].x+1,nowd=(nowd+1)%4;            }            else if(nowd==1)            {                int k = lower_bound(p+1,p+n+1,P(nowx,nowy),cmp1)-p-1;                if(k==0||p[k].x!=nowx)                {                    printf("%d\n",ans);                    break;                }                else ans++,nowy = p[k].y+1,nowd=(nowd+1)%4;            }            else            {                int k = lower_bound(p+1,p+n+1,P(nowx,nowy),cmp1)-p;                if(k==n+1||p[k].x!=nowx)                {                    printf("%d\n",ans);                    break;                }                else ans++,nowy = p[k].y-1,nowd=(nowd+1)%4;            }            if(ans>4*n)            {                printf("-1\n");                break;            }        }    }    return 0;}