#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <tuple>
#include <cstdint>
#include <cstdio>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <deque>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <cctype>
#include <random>
#include <cassert>
#include <numeric>
#define ll long long int
#define LL unsigned long long
#define pb push_back
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for(int i=1;i<=(n);i++)
using namespace std;

int mx8[] = {0,0,1,-1,-1,1,-1,1};
int my8[] = {-1,1,0,0,-1,-1,1,1};
int mx4[] = {1,-1,0,0};
int my4[] = {0,0,-1,1};
const int MOD = 1000000007;



int main() {
    int n,cnt=0; cin >> n;
    vector<int> t(n),ans;
    rep(i,n) cin >> t[i];
    rep(i,12){
        map<int,int> mp;
        mp[i%12]++;
        mp[(i+2)%12]++;
        mp[(i+4)%12]++;
        mp[(i+5)%12]++;
        mp[(i+7)%12]++;
        mp[(i+9)%12]++;
        mp[(i+11)%12]++;
        bool ok = true;
        rep(j,n){
            if(mp[t[j]]) continue;
            else {
                ok = false;//cout << t[j] << endl;
            }
        }
        if(ok){
            cnt++;
            ans.pb(i);
        }
    }
    //cout <<"cnt=" << cnt << endl;
    //cout << ans.size() << endl;
    if(cnt==1) cout << ans[0] << endl;
    else cout << -1 << endl;
}