import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n = ni()
b = na()
x = 0
for i in range(n):
    x |= b[i]

if all( (i + 1) & x == b[i] for i in range(n)):
    print(x)
else:
    print(-1)