/* -*- coding: utf-8 -*-
 *
 * 1109.cc:  No.1109 調の判定 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 12;
const bool ms[12] = { 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1 };

/* typedef */

/* global variables */

int ts[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", ts + i);

  int m = 0, d = -1;
  for (int i = 0; i < 12; i++) {
    bool ok = true;
    for (int j = 0; ok && j < n; j++) ok = ms[(ts[j] - i + 12) % 12];
    if (ok) m++, d = i;
  }

  printf("%d\n", (m == 1) ? d : -1);
  return 0;
}