from sys import stdin,setrecursionlimit#,set_int_max_str_digits
#import pypyjit
#pypyjit.set_param('max_unroll_recursion=-1')
# setrecursionlimit(2000000) # これこどふぉだと無理
# set_int_max_str_digits(200010)
mod = 998244353 
ii = lambda :int(stdin.readline())
mi = lambda :map(int,stdin.readline().split())
li = lambda :list(mi())
gmi = lambda :map(lambda x: int(x) - 1, stdin.readline().split())
gi = lambda :list(map(lambda x: 0 if x == "." else 1,input())) # グリッド入力受け取り
py = lambda :print("Yes")
pn = lambda :print("No")
pf = lambda :print("First") 
ps = lambda :print("Second")
pyn = lambda x: print("Yes") if x else print("No")
vec = [(1,0),(-1,0),(0,-1),(0,1)]
vec1 = [(1,0),(1,1),(0,1),(-1,1),(-1,0),(-1,-1),(0,-1),(1,-1)] #8方向
vec2 = [(1,1),(1,0),(0,-1),(-1,-1),(-1,0),(0,1)] # 六角形グリッド
inf = 10**18
from collections import defaultdict,deque
from heapq import heappop,heappush
# from random import randint
# from itertools import permutations 

B = 3 
n = ii() 
a = li() 

ans = 0 
memo = -1
for bit in range(B):
    tmp = 0 
    tmp1 = 0 
    ok = 0 
    for i,x in enumerate(a,1):
        ii = (i>>bit) & 1 
        xx = (x >> bit) & 1 
        if xx > ii:
            print(-1)
            exit()
        if ii == xx == 1:
            tmp1 = 1 
        if ii == 1 and xx == 0:
            tmp = 1 
        if ii == xx == 0:
            ok = 1 
    
    if tmp == tmp1 == 1:
        print(-1)
        exit()
    
    if tmp1 == 1:
        ans += 1<<bit 
        continue 
    if ok and memo < 0:
        memo = bit 

print(ans if ans else 1<<memo)