import sys

sys.setrecursionlimit(10**7)
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N = I()
A = [-1]+LI()

flag = [0]*(N+1)
if A[1] != A[-1]:
    color = -1
    for i in range(1,N+1):
        a = A[i]
        if a != color:
            if flag[a]:
                print(-1)
                break
            else:
                color = a
                flag[a] = 1
    else:
        print(0)
else:
    if A == [-1]+[A[1]]*N:
        print(0)
        exit()
    color = A[1]
    for i in range(1,N+1):
        a = A[i]
        if a != color:
            if flag[a]:
                print(-1)
                break
            else:
                color = a
                flag[a] = 1
    else:
        print(1)