#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

int main() {
    int n;
    cin >> n;

    int t[n];
    rep(i,n)cin >> t[i];

    int f[12];
    rep(i,12)f[i] = 1;

    int a[7] = {0,2,4,5,7,9,11};

    rep(i,12){
        rep(j,n){
            int flag = 0;
            rep(k,7){
                if(t[j] == (i + a[k]) % 12){
                    flag = 1;
                    break;
                }
            }
            if(flag == 0){
                f[i] = 0;
                break;
            }
        }
    }

    int arg = -1;
    rep(i,12){
        if(f[i]){
            if(arg != -1){
                cout << -1 << endl;
                return 0;
            }
            arg = i;
        }
    }

    cout << arg << endl;
    return 0;
}