#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  int t;
  cin >> t;
  for (int cs = 1; cs <= t; cs++) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    int sum = 0, odd = 0;
    string ans = "";
    for (int i = 0; i <= n - 1; i += 1) {
      int now = (s[i] - '0');
      if (now % 2) {
        odd++;
      }
      ans += s[i];
      if (odd > 0 and odd % 2 == 0) {
        break;
      }
    }
    if (odd == 0 or odd % 2) {
      cout << -1 << endl;
    } else {
      cout << ans << endl;
    }
  }
}
