import sys
input=sys.stdin.readline
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))

def main():
    mod=10**9+7
    N=I()
    T=LI()
    L=[0,2,4,5,7,9,11]
    
    F=[0]*12
    
    for d in range(12):
        flag=1
        for j in range(N):
            a=(T[j]-d)%12
            if not(a in L):
                flag=0
        if flag:
            F[d]=1
            
    if sum(F)==1:
        for i in range(12):
            if F[i]==1:
                print(i)
                exit()
    else:
        print(-1)
    


main()