#include <cstdlib>
#include <iostream>
#include <string>
#include <vector>

std::string states;
int n;
int num_queries_asked_in_this_round;

std::vector<int> query(const std::vector<int>& indices) {
  if (indices.size() != 2 * n) {
    std::cout << "-1\n";
    std::exit(0);
  }

  std::vector<bool> seen(2 * n + 1, false);
  for (int x : indices) {
    if (x < 1 || x > 2 * n || seen[x]) {
      std::cout << "-1\n";
      std::exit(0);
    }
    seen[x] = true;
  }

  num_queries_asked_in_this_round++;
  if (num_queries_asked_in_this_round > 2) {
    std::cout << "-1\n";
    std::exit(0);
  }

  std::vector<int> result(n);
  for (int i = 0; i < n; i++) {
    result[i] =
        states[indices[2 * i] - 1] - '0' + states[indices[2 * i + 1] - 1] - '0';
  }
  return result;
}

std::string solve(int n) {
  const auto sums = query({1, 2, 3, 4});
  return std::string(2 * n, '0');  // Placeholder implementation
}

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);

  int t;
  std::cin >> t;
  for (int i = 0; i < t; i++) {
    std::cin >> n >> states;

    num_queries_asked_in_this_round = 0;

    std::cout << solve(n) << "\n";
  }
  return 0;
}