#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int tc, n;
  string s;
  cin >> tc;
  while (tc--) {
    cin >> n >> s;
    string ans = "";
    bool odd = false, ada = false;
    for (int i = 0; i < n; i++) {
      if ((s[i] - '0') % 2 == 1 && !odd)
        odd = true, ans += s[i];
      else if ((s[i] - '0') % 2 == 1 && odd) {
        ada = true, ans += s[i];
        break;
      }
    }
    if (ada)
      cout << ans << '\n';
    else
      cout << "-1" << '\n';
  }
}
