n=int(input())
arr=list(map(int,input().split()))
if n==1:
    print(0)
    exit()
if arr.count(arr[0])==n:
    print(0)
    exit()
if arr[0]==arr[-1]:
    s=set()
    s.add(arr[0])
    for i in range(n):
        if arr[i]==arr[0]:
            continue
        else:
            arr=arr[i:]
            break
    for i in range(n):
        if arr[-(i+1)]==arr[-1]:
            continue
        else:
            arr=arr[:-i]
            break
    n=len(arr)
    arr.append(0)
    for i in range(n):
        if arr[i] in s:
            print(-1)
            break
        if arr[i]==arr[i+1]:
            continue
        else:
            s.add(arr[i])
    else:
        print(1)
else:
    s=set()
    arr.append(0)
    for i in range(n):
        if arr[i] in s:
            print(-1)
            break
        if arr[i]==arr[i+1]:
            continue
        else:
            s.add(arr[i])
    else:
        print(0)