#include <bits/stdc++.h>
using namespace std;
int main() {
  long long t;
  cin >> t;
  while (t--) {
    long long n, flag = 0;
    long long res = 0;
    cin >> n;
    string str;
    string s;
    s = "";
    cin >> str;
    for (long long i = 0; i < n; i++) {
      if ((str[i] - '0') % 2 != 0) {
        s.push_back(str[i]);
        flag++;
      }
      if (flag == 2) break;
    }
    if (flag != 2)
      cout << "-1" << endl;
    else
      cout << s << endl;
  }
  return 0;
}
