def main():
    n = int(input())
    p = list(map(int, input().split()))
    ans = 0
    flag = True
    while flag:
        flag = False
        for i in range(n-1):
            if p[i] > p[i+1]:
                p[i], p[i+1] = p[i+1], p[i]
                ans += 1
                flag = True

    for i in range(n-1):
        if p[i] == p[i+1]:
            return -1
    return ans

print(main())