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