#!/usr/bin/env python3

import sys
input=sys.stdin.readline

n=int(input())
arr=list(map(int,input().split()))
scale=[0,2,4,5,7,9,11]
cand=[]
for i in range(12):
    s=set()
    for j in range(7):
        s.add((scale[j]+i)%12)
    for val in arr:
        if val not in s:
            break
    else:
        cand.append(i)
if len(cand)==1:
    print(cand[0])
else:
    print(-1)