#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int t;
  cin >> t;
  while (t--) {
    long long int n;
    cin >> n;
    string s;
    cin >> s;
    string a;
    long long int i;
    for (i = 0; i < n; i++) {
      int b = (int)s[i];
      if (s[i] % 2 == 1) a.push_back(s[i]);
      if (a.length() == 2) {
        cout << a << endl;
        break;
      }
    }
    if (a.length() != 2) cout << -1 << endl;
  }
}
