import sys
input = sys.stdin.readline
N = int(input())
B = list(map(int, input().split()))
X = [-1 for _ in range(20)]
for j in range(N):
    i = j+1
    b = B[j]
    for k in range(20):
        ii = (i>>k)&1
        bb = (b>>k)&1
        if ii==1:
            if bb==0:
                if X[k]==1:
                    print(-1)
                    exit()
                else:
                    X[k] = 0
            else:
                if X[k]==0:
                    print(-1)
                    exit()
                else:
                    X[k] = 1
        else:
            if bb==1:
                print(-1)
                exit()
ans = 0
for i in range(20):
    if X[i]==1:
        ans |= 1<<i
print(ans)