#include <bits/stdc++.h>
using namespace std;
long long nCr(long long n, long long r) {
  vector<vector<long long> > ans(n + 1);
  ans[0].push_back(1);
  ans[1].push_back(1);
  ans[1].push_back(1);
  for (int i = 2; i <= n; i++) {
    ans[i].push_back(1);
    for (int j = 1; j < ans[i - 1].size(); j++) {
      ans[i].push_back((ans[i - 1][j - 1] + ans[i - 1][j]));
    }
    ans[i].push_back(1);
  }
  return ans[n][r];
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int t, i, n;
  cin >> t;
  while (t--) {
    string s, a = "";
    cin >> n >> s;
    for (i = 0; i < n; i++) {
      if ((s[i] - '0') % 2 != 0) a += s[i];
    }
    if (a.size() <= 1)
      cout << -1 << endl;
    else {
      if (a.size() % 2 != 0) {
        a.erase(a.begin() + a.size() - 1);
        cout << a << endl;
      } else
        cout << a << endl;
    }
  }
  return 0;
}
