#include <bits/stdc++.h>
#pragma GCC optimize("O2")
#pragma GCC optimize("unroll-loops")
using namespace std;
const int MAX = 2e5 + 5;
const long long MAX2 = 11;
const long long MOD = 1000000007;
const long long MOD2 = 1000005329;
const long long INF = 2e18;
const int dr[] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const int dc[] = {0, 1, 0, -1, 1, -1, 1, -1, 0};
const double pi = acos(-1);
const double EPS = 1e-9;
const int block = 555;
int tc, n, x[MAX], y[MAX], nx, nw, hit;
bool vis[MAX], vs[MAX], st[MAX], ava;
vector<int> v[MAX], ans, lp;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> tc;
  for (int ntc = 1; ntc <= tc; ++ntc) {
    cin >> n;
    for (int i = 1; i <= n; ++i) v[i].clear(), vis[i] = vs[i] = st[i] = 0;
    for (int i = 1; i <= n; ++i) cin >> x[i], v[x[i]].push_back(i);
    for (int i = 1; i <= n; ++i) cin >> y[i], v[y[i]].push_back(i);
    ava = 1;
    for (int i = 1; i <= n; ++i)
      if (v[i].size() != 2) ava = 0;
    if (!ava) {
      cout << "-1\n";
      continue;
    }
    ans.clear();
    for (int i = 1; i <= n; ++i)
      if (!vis[i]) {
        lp.clear(), nw = i, hit = 0;
        while (!vis[nw]) {
          vis[nw] = 1;
          lp.push_back(nw);
          if (vs[x[nw]]) {
            st[nw] = vs[y[nw]] = 1, ++hit;
            nw = nw == v[y[nw]][0] ? v[y[nw]][1] : v[y[nw]][0];
          } else
            vs[x[nw]] = 1, nw = nw == v[x[nw]][0] ? v[x[nw]][1] : v[x[nw]][0];
        }
        if (hit > lp.size() - hit) {
          for (int j : lp)
            if (!st[j]) ans.push_back(j);
        } else {
          for (int j : lp)
            if (st[j]) ans.push_back(j);
        }
      }
    cout << ans.size() << '\n';
    for (int i : ans) cout << i << ' ';
    cout << '\n';
  }
  return 0;
}
