N=int(input())
B=list(map(int,input().split()))
x=0
for i,b in enumerate(B,1):
    if b>i:exit(print(-1))
    x|=b
for i,b in enumerate(B,1):
    if i&x!=b:exit(print(-1))
if x==0:print(2**(len(bin(N)[2:])))
else:print(x)