#include <bits/stdc++.h>
#define ll long long
using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define rrep(i, n) for (int i = 1; i <= n; i++)
ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
ll lcm(ll a, ll b) { return (a * b) / gcd(a,b); }
using P = pair <int, int>;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
 
ll MOD = 1000000007;
ll INF =1<<30;
ll dp[110][100100];
 

int main(){
   int n;
   cin >> n;
   vector<int>a(n);
   rep(i,n)cin >> a[i];
   vector<int> ok(n,0);
   int cnt = 0;
   int d;
   rep(j,12){
      rep(i,n) ok[i] =0;
      rep(i,n){
         if(a[i]==j%12||a[i]==(j+2)%12||a[i]==(j+4)%12||a[i]==(j+5)%12||a[i]==(j+7)%12||a[i]==(j+9)%12||
         a[i]==(j+11)%12)
         ok[i]++;
         }
      bool p = true;
      rep(i,n){
         if(ok[i]!=1){
            p=false;
         }
      }
      if(p){
         cnt ++ ;
         d = j;
      }

   }
   if(cnt==1) cout << d<<endl;
   else cout << -1 <<endl;
}