import sys
sys.stdin.readline
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))
mod=10**9+7

def main():
    N=I()
    A=LI()
    now=0
    use=[0]*(N)
    for i in range(N):
        if now!=A[i]:
            use[A[i]-1]+=1
        now=A[i]
        
    if max(use)<=1:
        print(0)
    else:
        if A[0]==A[-1]:
            use[A[0]-1]-=1
        
        if max(use)<=1:
            print(1)
        else:
            print(-1)
    
    


main()