#include <bits/stdc++.h> //全てのヘッダファイルをインクルード

//ループ
#define rep(i, n) for(int i = 0; i < (n); i++) //普通のループ
#define repr(i, n) for(int i = n; i >= 0; i--) //逆ループ

//型名省略
typedef long long ll;
//値
static const ll MX = 100005;
static const ll MX_ll = 1e18;

using namespace std;

//#include "./lib/generic/search.h"

int main(){
    int n;
    cin >> n;
    int t[n], chk[7], ans=0, cnt=0, cnt2=0;
    rep(i, n){
        cin >> t[i];
        chk[i] = 0;
    }
    rep(i, n){
        cnt = 0;
        chk[0] = t[i]%12;
        chk[1] = (t[i]+2)%12;
        chk[2] = (t[i]+4)%12;
        chk[3] = (t[i]+5)%12;
        chk[4] = (t[i]+7)%12;
        chk[5] = (t[i]+9)%12;
        chk[6] = (t[i]+11)%12;
        rep(j, n){
            rep(k, 7){
                if(t[j]==chk[k]) cnt++;
            }
        }
        if(cnt==n && cnt2==0){
            cnt2 = 1;
            ans = t[i];
            cnt = 0;
        }
        if(cnt==n && cnt2==1){
            cnt2 = 2;
            ans = -1;
            break;
        }
    }
    if(cnt2==1) cout << ans << endl;
    else cout << "-1" << endl;
    return 0;
}