N=int(input())
T=list(map(int,input().split()))
X=[0,2,4,5,7,9,11]
l=[]
for i in range(0,12):
    sw=0
    for j in T:
        if (j-i)%12 in X:
            pass
        else:
            sw=1
            break
    if sw==0:
        l.append(i)
if len(l)==1:
    print(l[0])
else:
    print(-1)