import sys
from functools import reduce
from math import gcd


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    n = input()
    t = set(map(int, input().split()))
    ans = []
    for d in range(12):
        s = {d, (d + 2) % 12, (d + 4) % 12, (d + 5) % 12, (d + 7) % 12, (d + 9) % 12, (d + 11) % 12}
        if s.issuperset(t):
            ans.append(d)
    if len(ans) != 1:
        return -1
    return ans[0]


if __name__ == '__main__':
    ret = main()
    if ret is not None:
        print(ret)