import sys
input=lambda: sys.stdin.readline().rstrip()
n=int(input())
T=[int(i) for i in input().split()]
D=[0,2,4,5,7,9,11]
Ans=[]
for i in range(12):
  for t in T:
    if (t-i)%12 not in D:
      break
  else:
    Ans.append(i)
if len(Ans)!=1:
  print(-1)
else:
  print(Ans[0]%12)