import sys, re
from fractions import Fraction
from math import ceil, floor, sqrt, pi, factorial, gcd
from copy import deepcopy
from collections import Counter, deque, defaultdict
from heapq import heapify, heappop, heappush
from itertools import accumulate, product, combinations, combinations_with_replacement, permutations
from bisect import bisect, bisect_left, bisect_right
from functools import reduce
from decimal import Decimal, getcontext, ROUND_HALF_UP
def i_input(): return int(input())
def i_map(): return map(int, input().split())
def i_list(): return list(i_map())
def i_row(N): return [i_input() for _ in range(N)]
def i_row_list(N): return [i_list() for _ in range(N)]
def s_input(): return input()
def s_map(): return input().split()
def s_list(): return list(s_map())
def s_row(N): return [s_input for _ in range(N)]
def s_row_str(N): return [s_list() for _ in range(N)]
def s_row_list(N): return [list(s_input()) for _ in range(N)]
def lcm(a, b): return a * b // gcd(a, b)
def get_distance(x1, y1, x2, y2):
	  d = sqrt((x2 - x1) ** 2 + (y2 - y1) ** 2)
	  return d
def rotate(table):
   	  n_fild = []
   	  for x in zip(*table[::-1]):
   	  	  n_fild.append(x)
   	  return n_fild
sys.setrecursionlimit(10 ** 7)
INF = float('inf')
MOD = 10 ** 9 + 7
MOD2 = 998244353


def main():
   
   N = int(input())
   A = i_list()
   ok = [0, 2, 4, 5, 7, 9, 11]
   
   ans = []
   for i in range(12):
   	  juge = True
   	  for j in range(N):
   	  	  p = (A[j] - i) % 12 
   	  	  if p not in ok:
   	  	  	  juge = False
   	  	  	  break
   	  if juge:
   	  	  ans.append(i)
   
   if len(ans) == 1:
   	  print(ans[-1])
   else:
   	  print(-1)
   
if __name__ == '__main__':
    main()