N = int(input())
T = [int(t) for t in input().split()]
mod = 12

ans = []
for i in range(N):
    c = [0,2,4,5,7,9,11]
    for j in range(7):
        c[j] += T[i]
        c[j] = c[j]%mod
    #print(c)
    cnt = 0
    for t in T:
        if t in c:
            cnt += 1
    if cnt == N:
        ans.append(T[i])
ans = set(ans)

if len(ans) == 1:
    print(*ans)
else:
    print(-1)