#include <bits/stdc++.h>
using namespace std;
int n, a[2][200005], cnt, e[200005], z, minim;
bool viz[200005];
vector<pair<int, int> > Muchii[200005];
void dfs(int nod, int i) {
  e[++z] = i;
  viz[i] = true;
  for (auto it : Muchii[nod]) {
    if (viz[it.second] == false) {
      dfs(it.first, it.second);
    }
  }
}
void solve() {
  vector<int> ans;
  cin >> n;
  vector<int> c(n + 3);
  bool ok = true;
  for (int i = 0; i < 2; ++i)
    for (int j = 1; j <= n; ++j) {
      cin >> a[i][j];
      c[a[i][j]]++;
    }
  for (int i = 1; i <= n; ++i) {
    if (c[i] != 2) {
      ok = false;
      break;
    }
  }
  if (ok == false) {
    cout << -1 << "\n";
    return;
  }
  for (int i = 1; i <= n; ++i) {
    Muchii[a[0][i]].push_back({a[1][i], i});
    Muchii[a[1][i]].push_back({a[0][i], i});
  }
  for (int i = 1; i <= n; ++i) {
    if (a[0][i] == a[1][i]) continue;
    if (viz[i] == false) {
      z = 0;
      dfs(a[1][i], i);
      ++cnt;
      int fr1 = 0, fr2 = 1;
      pair<int, int> last = {a[0][e[1]], a[1][e[1]]};
      for (int j = 2; j <= z; ++j) {
        if (last.first == a[0][e[j]] || last.second == a[1][e[j]]) {
          ++fr1;
          last = {a[1][e[j]], a[0][e[j]]};
        } else {
          last = {a[0][e[j]], a[1][e[j]]};
        }
      }
      last = {a[1][e[1]], a[0][e[1]]};
      for (int j = 2; j <= z; ++j) {
        if (last.first == a[0][e[j]] || last.second == a[1][e[j]]) {
          ++fr2;
          last = {a[1][e[j]], a[0][e[j]]};
        } else {
          last = {a[0][e[j]], a[1][e[j]]};
        }
      }
      minim += min(fr1, fr2);
      if (fr1 > fr2) swap(a[0][e[1]], a[1][e[1]]), ans.push_back(e[1]);
      for (int j = 2; j <= z; ++j) {
        if (a[0][e[j - 1]] == a[0][e[j]] || a[1][e[j - 1]] == a[1][e[j]])
          swap(a[0][e[j]], a[1][e[j]]), ans.push_back(e[j]);
      }
    }
  }
  cout << ans.size() << "\n";
  for (int i = 0; i < ans.size(); ++i) {
    cout << ans[i] << " ";
  }
  cout << "\n";
  for (int i = 1; i <= n; ++i) {
    Muchii[i].clear();
    viz[i] = false;
  }
}
int main() {
  int t;
  cin >> t;
  while (t--) solve();
  return 0;
}
