def min_trip(arr):
    mp = {}

    for x in arr:
        mp[x] = mp.get(x, 0) + 1

    trips = 0

    for count in mp.values():
        if count == 1:
            return -1

        trips += count // 3

        if count % 3 != 0:
            trips += 1

    return trips


n = int(input())
arr = list(map(int, input().split()))

print(min_trip(arr))