N = int(input())
T = list(map(int,input().split()))
C = [0,2,4,5,7,9,11]
cands = []
for i in range(12):
    A = [(c+i)%12 for c in C]
    if all(t in A for t in T):
        cands.append(i)

print(cands[0] if len(cands)==1 else -1)