import sys
sys.setrecursionlimit(10**7) #再帰関数の上限,10**5以上の場合python
import math
from copy import copy, deepcopy
from copy import deepcopy as dcp
from operator import itemgetter
from bisect import bisect_left, bisect, bisect_right#2分探索
#bisect_left(l,x), bisect(l,x)#aはソート済みである必要あり。aの中からx未満の要素数を返す。rightだと以下
from collections import deque
#deque(l), pop(), append(x), popleft(), appendleft(x)
#q.rotate(n)で → にn回ローテート
from collections import Counter#文字列を個数カウント辞書に、
#S=Counter(l),S.most_common(x),S.keys(),S.values(),S.items()
from itertools import accumulate,combinations,permutations#累積和
#list(accumulate(l))
from heapq import heapify,heappop,heappush
#heapify(q),heappush(q,a),heappop(q) #q=heapify(q)としないこと、返り値はNone
#import fractions#古いatcoderコンテストの場合GCDなどはここからimportする
from functools import lru_cache#pypyでもうごく
#@lru_cache(maxsize = None)#maxsizeは保存するデータ数の最大値、2**nが最も高効率
from decimal import Decimal

def input(): 
    x=sys.stdin.readline()
    return x[:-1] if x[-1]=="\n" else x
def printl(li): _=print(*li, sep="\n") if li else None
def argsort(s, return_sorted=False): 
    inds=sorted(range(len(s)), key=lambda k: s[k])
    if return_sorted: return inds, [s[i] for i in inds]
    return inds
def alp2num(c,cap=False): return ord(c)-97 if not cap else ord(c)-65
def num2alp(i,cap=False): return chr(i+97) if not cap else chr(i+65)
def matmat(A,B):
    K,N,M=len(B),len(A),len(B[0])
    return [[sum([(A[i][k]*B[k][j]) for k in range(K)]) for j in range(M)] for i in range(N)]
def matvec(M,v):
    N,size=len(v),len(M)
    return [sum([M[i][j]*v[j] for j in range(N)]) for i in range(size)]
def T(M):
    n,m=len(M),len(M[0])
    return [[M[j][i] for j in range(n)] for i in range(m)]
def main():
    mod = 1000000007
    #w.sort(key=itemgetter(1),reversed=True)  #二個目の要素で降順並び替え

    N = int(input())
    #N, K = map(int, input().split())
    A = tuple(map(int, input().split())) #1行ベクトル
    #L = tuple(int(input()) for i in range(N)) #改行ベクトル
    #S = tuple(tuple(map(int, input().split())) for i in range(N)) #改行行列
    ref=0
    for i in range(2,N):
        ref^=A[i]
    
    a0=A[0]
    a1=A[1]
    s=a0+a1

    # ans=0
    # #print(bin(a0),bin(a1),bin(ref))
    # INF=10**13
    # @lru_cache(None)
    # def dfs(a0,a1,ref):
    #     inf=INF
    #     if (a0 & 1) ^ (a1 & 1) != (ref & 1):
    #         return inf
        
    #     if a0<0:
    #         return inf
    #     if ref == 0:
    #         return inf if a0<a1 else (a0-a1)//2
    #     a01=a0&1
    #     a11=a1&1
    #     ref1=ref&1
    #     ans=inf
    #     if a01^a11==ref1:
    #         ans=min(ans,2*dfs(a0>>1,a1>>1,ref>>1))
    #     ans=min(ans,2*dfs((a0-1)//2,(a1+1)//2,ref>>1)+1)

    #     return ans
    # x=dfs(a0,a1,ref)
    # if x>=a0:
    #     print(-1)
    
    ##桁dp
    dp=[[[-1]*2 for _ in range(2)] for _ in range(43)]
    refi=ref
    si=s
    dp[0][0][0]=0
    shift=1
    a=a0
    for i in range(42):
        ni=i+1
        for j in range(2):
            for k in range(2):
                cur=dp[i][j][k]
                if cur==-1:
                    continue
                for ai in range(0,2):
                    bi=(refi&1)^ai
                    if (bi+j+ai)&1!=si&1:
                        continue
                    if ai>a&1:
                        nk=1
                    elif ai==a&1:
                        nk=k
                    else:
                        nk=0

                    nj=0
                    if bi+j+ai>1:
                        nj=1
                    
                    dp[ni][nj][nk]=max(dp[ni][nj][nk],cur|shift*ai)
                    
        refi=(refi>>1)
        si>>=1
        a>>=1
        shift<<=1
    x=dp[-1][0][0]
    if x==-1 or x==0:
        print(-1)
    else:
        print(a0-x)


                





if __name__ == "__main__":
    main()

Measured memory_bytes: 10100