from copy import deepcopy

N=int(input())
T=list(map(int,input().split()))

G=[]
for i in range(12):
    G.append([i,(i+2)%12,(i+4)%12,(i+5)%12,(i+7)%12,(i+9)%12,(i+11)%12])

for t in T:
    V=[]
    for H in G:
        if t in H:
            V.append(H)

    G=deepcopy(V)

if len(G)!=1:
    print(-1)
else:
    print(G[0][0])