mod = 1000000007
eps = 10**-9


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

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

    pos = [[] for _ in range(N+1)]
    for i, a in enumerate(A):
        pos[a].append(i)

    flg = 0
    for a in range(N):
        if pos[a]:
            L = len(pos[a])
            prev = pos[a][0]
            if prev != 0:
                for i in pos[a][1:]:
                    if i != prev + 1:
                        print(-1)
                        exit()
                    else:
                        prev = i
            else:
                for i in range(1, L):
                    ii = pos[a][i]
                    if ii != prev + 1:
                        flg = 1
                        j = i-1
                        break
                    else:
                        prev = ii
                if flg:
                    prev = N
                    for i in range(L-1, j, -1):
                        ii = pos[a][i]
                        if ii != prev - 1:
                            print(-1)
                            exit()
                        else:
                            prev = ii
    print(flg)


if __name__ == '__main__':
    main()