n = int(input())
t = [int(i) for i in input().split()]

cho = [[0]*7 for i in range(12)]
c = [0,2,4,5,7,9,11]
for i in range(12):
    for j in range(7):
        cho[i][j] = (i+c[j])%12

ans = -1

for i in range(12):
    F = True
    for j in range(n):
        f = False
        for k in range(7):
            if t[j] == cho[i][k]:
                f = True
                break
        if f == False:
            F = False
            break
    if f == True:
        if ans != -1:
            print(-1)
            exit()
        ans = i
print(ans)