#include <bits/stdc++.h>
using namespace std;
void print(vector<int> v) {
  for (int i = 0; i < v.size(); i++) {
    if (v[i] != 0) {
      cout << v[i];
    }
  }
  cout << endl;
}
void solve(vector<int>& v, int even, int odd) {
  if (v.size() == 1) {
    cout << -1 << endl;
    return;
  } else if (v.size() == 2) {
    if (odd != 2) {
      cout << -1 << endl;
      return;
    } else {
      print(v);
      return;
    }
  } else if (odd == 0) {
    cout << -1 << endl;
    return;
  } else {
    while (v[v.size() - 1] % 2 == 0) {
      v.pop_back();
      even--;
    }
    if (odd == 1) {
      cout << -1 << endl;
      return;
    } else if (odd % 2 == 0) {
      print(v);
      return;
    } else {
      for (int i = 0; i < v.size(); i++) {
        if (v[i] % 2 == 1) {
          v.erase(v.begin() + i);
          print(v);
          return;
        }
      }
    }
  }
}
int main() {
  int t;
  cin >> t;
  while (t--) {
    int e = 0, o = 0;
    int n;
    cin >> n;
    string s;
    cin >> s;
    vector<int> v(n);
    for (int i = 0; i < s.size(); i++) {
      v[i] = s[i] - 48;
      if (v[i] % 2 == 0)
        e++;
      else
        o++;
    }
    solve(v, e, o);
  }
}
