mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    N = int(input())
    A = list(map(int, input().split()))

    AA = []
    prev = -1
    for a in A:
        if a != prev:
            AA.append(a)
        prev = a
    seen = [0] * (N+1)
    for i in range(len(AA)-1):
        a = AA[i]
        if seen[a]:
            print(-1)
            exit()
        seen[a] = 1
    if seen[AA[-1]]:
        print(1)
    else:
        print(0)


if __name__ == '__main__':
    main()