from sys import stdin
def check1(n,a):
    s=set()
    for i in range(n):
        if i==0:
            m=a[i]
        else:
            if m!=a[i]:
                if a[i] in s:
                    break
                else:
                    s.add(m)
                    m=a[i]
    else:
        return True
    return False

def  check2(n,a):
    for i in range(n):
        if i==0:
            m=a[i]
        else:
            if m!=a[i]:
                j=i
                break
    flag=True
    for i in range(n-1,-1,-1):
        if i==n-1:
            if m!=a[i]:
                flag=False
                break
        else:
            if m!=a[i]:
                k=i+1
                break

    if flag:
        s={m}
        return check3(len(a[j:k]),a[j:k],s)
    else:
        return False

def check3(n,a,s):
    for i in range(n):
        if i==0:
            m=a[i]
        else:
            if m!=a[i]:
                if a[i] in s:
                    break
                else:
                    s.add(m)
                    m=a[i]
    else:
        return True
    return False

def main():
    #入力
    readline=stdin.readline
    n=int(readline())
    a=list(map(int,readline().split()))

    if check1(n,a): print(0)
    elif check2(n,a): print(1)
    else: print(-1)

if __name__=="__main__":
    main()