#include <bits/stdc++.h>
using namespace std;
bool RELAXED = false;
template <int, typename T>
struct MINMAX {
  T val;
  MINMAX(T val) : val(val) {}
};
template <typename T>
MINMAX<1, T> MAX(T val) {
  return MINMAX<1, T>(val);
};
template <typename T>
MINMAX<2, T> MIN(T val) {
  return MINMAX<2, T>(val);
};
template <typename T, typename U>
inline T& operator|=(T& lhs, MINMAX<1, U> rhs) {
  return lhs = (rhs.val > lhs ? (RELAXED = true, rhs.val)
                              : (RELAXED = false, lhs));
}
template <typename T, typename U>
inline T& operator|=(T& lhs, MINMAX<2, U> rhs) {
  return lhs = (rhs.val < lhs ? (RELAXED = true, rhs.val)
                              : (RELAXED = false, lhs));
}
template <typename T>
inline vector<T> readVector(int n) {
  vector<T> vec(n);
  for (int i = 0; i < int(n); i++) cin >> vec[i];
  return vec;
}
template <typename T>
void EXIT(T answer) {
  cout << answer << endl;
  exit(0);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  int T;
  cin >> T;
  while (T--) {
    int n;
    cin >> n;
    string s;
    cin >> s;
    string t;
    for (auto c : s) {
      if ((c - '0') % 2) t.push_back(c);
    }
    if (t.length() >= 2)
      cout << t.substr(0, 2) << endl;
    else
      cout << -1 << endl;
  }
  return 0;
}
