#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define dwn(i,a,b) for(int i=(a);i>=(b);i--)
using namespace std;
const int maxn=50050;
int maxx(int a,int b){return a<b?b:a;}

int n,Ans,a[2*maxn],b[maxn],ls[maxn],nd[maxn][2],nt[maxn][2],ji[maxn];
int vis[4*maxn];
void worrk(){
    memset(vis,0,sizeof(vis));
    rep(i,1,n)ls[b[i]]=i;//离散化，将目标序列映射成1-n
    rep(i,1,n)a[i]=ls[a[i]];
    rep(i,1,n)a[i+n]=a[i];//断链为环
    int re=a[1]-1,ji=0;
    int ans=0;
    rep(i,1,n)vis[2*n+a[i]-i]++;
    ans=max(ans,vis[2*n+0]);
    rep(k,1,n){
        vis[2*n+a[n+k]-n-k]++;//和下标做差，若平移了k次，那么就要统计-k的个数
        vis[2*n+a[k]-k]--;
        ans=max(ans,vis[2*n-k]);
    }
    Ans=min(Ans,n-ans);
}
int main(){
    scanf("%d",&n);
    rep(i,1,n){
        int a,b;
        scanf("%d%d",&a,&b);
        nd[i][0]=a;
        nd[i][1]=b;
        if(a==b||a==i||b==i){
            cout<<-1;
            return 0;
        }
        if(ji[a]>=2||ji[b]>=2){
            cout<<-1;
            return 0;
        }
        nt[a][ji[a]++]=i;
        nt[b][ji[b]++]=i;
    }
    rep(i,1,n){
        int pd=0;
        if(nd[i][0]==nt[i][0]&&nd[i][1]==nt[i][1])pd=1;
        if(nd[i][0]==nt[i][1]&&nd[i][1]==nt[i][0])pd=1;
        if(pd==0){
            cout<<-1;
            return 0;
        }
    }
    Ans=2000000;
    rep(i,1,n)
        a[i]=i;
    b[1]=1;b[2]=nt[1][0];b[n]=nt[1][1];
    rep(i,3,n-1){
        if(b[i-2]==nt[b[i-1]][0])b[i]=nt[b[i-1]][1];
        else b[i]=nt[b[i-1]][0];
    }
    worrk();

    rep(i,1,n)
        a[i]=i;
    b[1]=1;b[2]=nt[1][1];b[n]=nt[1][0];
    rep(i,3,n-1){
        if(b[i-2]==nt[b[i-1]][0])b[i]=nt[b[i-1]][1];
        else b[i]=nt[b[i-1]][0];
    }
    worrk();
    cout<<Ans;
    return 0;
}