#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007LL;
int t;
int n;
int a[2][200005];
int par[200005];
vector<int> cols[200005];
vector<int> edges[200005];
int col[200005];
int cnt[200005];
bool rev[200005];
bool dfs(int u, int c, vector<int>& v0, vector<int>& v1) {
  if (col[u] >= 0) {
    if (col[u] != c) return false;
    return true;
  }
  col[u] = c;
  if (c == 0) {
    v0.push_back(u);
  } else {
    v1.push_back(u);
  }
  for (int i = 0; i < edges[u].size(); ++i) {
    int v = edges[u][i];
    if (!dfs(v, !c, v0, v1)) return false;
  }
  return true;
}
int find(int u) { return u == par[u] ? u : par[u] = find(par[u]); }
int main() {
  cin >> t;
  for (int cas = 1; cas <= t; ++cas) {
    cin >> n;
    for (int i = 1; i <= n; ++i) {
      cols[i].clear();
      edges[i].clear();
    }
    for (int i = 0; i < 2; ++i) {
      for (int j = 1; j <= n; ++j) {
        scanf("%d", &a[i][j]);
        cols[a[i][j]].push_back(j);
      }
    }
    bool ok = true;
    for (int i = 1; i <= n; ++i) {
      if (cols[i].size() != 2) {
        ok = false;
        break;
      }
    }
    if (!ok) {
      cout << -1 << endl;
      continue;
    }
    for (int i = 1; i <= n; ++i) {
      par[i] = i;
      cnt[i] = 1;
      col[i] = -1;
      rev[i] = false;
    }
    for (int i = 1; i <= n; ++i) {
      int c1 = cols[i][0];
      int c2 = cols[i][1];
      if ((a[0][c1] == i && a[1][c2] == i) ||
          (a[1][c1] == i && a[0][c2] == i)) {
        int p1 = find(c1);
        int p2 = find(c2);
        if (p1 != p2) {
          par[p1] = p2;
          cnt[p2] += cnt[p1];
        }
      }
    }
    for (int i = 1; i <= n; ++i) {
      int c1 = cols[i][0];
      int c2 = cols[i][1];
      if (c1 == c2) continue;
      if ((a[0][c1] == i && a[0][c2] == i) ||
          (a[1][c1] == i && a[1][c2] == i)) {
        int p1 = find(c1);
        int p2 = find(c2);
        if (p1 == p2) {
          ok = false;
          break;
        }
        edges[p1].push_back(p2);
        edges[p2].push_back(p1);
      }
    }
    if (!ok) {
      cout << -1 << endl;
      continue;
    }
    for (int i = 1; i <= n; ++i) {
      par[i] = find(i);
      if (par[i] == i && col[i] < 0) {
        vector<int> v0, v1;
        v0.clear();
        v1.clear();
        if (!dfs(i, 0, v0, v1)) {
          ok = false;
          break;
        }
        int n0 = 0, n1 = 0;
        for (int j = 0; j < v0.size(); ++j) {
          n0 += cnt[v0[j]];
        }
        for (int j = 0; j < v1.size(); ++j) {
          n1 += cnt[v1[j]];
        }
        if (n0 < n1) {
          for (int j = 0; j < v0.size(); ++j) {
            rev[v0[j]] = true;
          }
        } else {
          for (int j = 0; j < v1.size(); ++j) {
            rev[v1[j]] = true;
          }
        }
      }
    }
    if (!ok) {
      cout << -1 << endl;
    } else {
      vector<int> ans;
      for (int i = 1; i <= n; ++i) {
        par[i] = find(i);
        if (rev[par[i]]) {
          ans.push_back(i);
        }
      }
      cout << ans.size() << endl;
      for (int i = 0; i < ans.size(); ++i) {
        printf("%d", ans[i]);
        if (i < ans.size() - 1)
          cout << " ";
        else
          cout << endl;
      }
    }
  }
  return 0;
}
