#include<bits/stdc++.h>
using namespace std;
const int maxn=2e5+100; 
int t;
int n;
int a[maxn];
int b[maxn];
vector<int> g[maxn];
int visit[maxn];
vector<int> wjm;
int main () {
    scanf("%d",&t);
    while (t--) {
        scanf("%d",&n);
        for (int i=1;i<=n;i++) g[i].clear();
        wjm.clear();
        memset(visit,
 
0,sizeof(visit));
        for (int i=1;i<=n;i++) {
            scanf("%d",&a[i]);
            g[a[i]].push_back(i);
        }
        for (int i=1;i<=n;i++) {
            scanf("%d",&b[i]);
            g[b[i]].push_back(i);
        }
        bool f=true;
        for (int i=1;i<=n;i++) if
 
 (g[i].size()!=2) f=false;
        if (!f) {
            printf("-1\n");
            continue;
        }
        for (int i=1;i<=n;i++) {
            if (visit[i]) continue;//如果已經被換過了就不做調整 
            if (g[i][0]==g[i][1]) continue;//如果位置一樣就不做調整 
            vector<int> x,y;
            for (int
 
 j=i,k=g[i][0];!visit[j];) {
                visit[j]=1;
                k=g[j][0]^g[j][1]^k;//高階操作，俺矇蔽 
                if (a[k]==j) {
                    x.push_back(k);
                    j=b[k];
                }
                else {
                    y.push_back(k);
                    j=a[k];
                }
            }
            if (x.size()<y.size()) 
                wjm.insert(wjm.end(),x.begin(),x.end());
            else
                wjm.insert(wjm.end(),y.begin(),y.end());
        }
        printf("%d\n",wjm.size());
        for (auto i:wjm) printf("%d ",i);
        printf("\n");
    }
}