#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-9;
int fx[] = {-1, 1, 0, 0, -1, -1, 1, 1};
int fy[] = {0, 0, -1, 1, -1, 1, -1, 1};
int kx[] = {1, 1, 2, 2, -1, -1, -2, -2};
int ky[] = {2, -2, 1, -1, 2, -2, 1, -1};
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int T;
  cin >> T;
  while (T--) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    string ans = "";
    for (int i = 0; i < n; i++) {
      if ((s[i] - '0') & 1) ans += s[i];
    }
    if (ans.size() & 1) ans.pop_back();
    if (!ans.size())
      cout << "-1\n";
    else
      cout << ans << "\n";
  }
  return 0;
}
