a.cpp

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pii;
template <typename T1, typename T2>
ostream &operator <<(ostream &os, pair<T1, T2> p){os << p.first << " " << p.second; return os;}
template <typename T>
ostream &operator <<(ostream &os, vector<T> &v){for(T i : v)os << i << ", "; return os;}
template <typename T>
ostream &operator <<(ostream &os, set<T> s){for(T i : s) os << i << ", "; return os;}
template <typename T1, typename T2>
ostream &operator <<(ostream &os, map<T1, T2> m){for(pair<T1, T2> i : m) os << i << endl; return os;}

int main(){
    int t; cin >> t;
    while(t--){
        int n; cin >> n;
        string s; cin >> s;
        vector<int> o;
        for(int i = 0; i < n; i++){
            if((int)s[i] % 2 == 1){
                o.push_back(i);    
            }
            if((int)o.size() == 2){
                break;
            }
        }
        if(o.size() < 2) cout << -1 << endl;
        else cout << s[o[0]] << s[o[1]] << endl;
    }
    return 0;
}