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

count = [0] * (n + 1)
for num in a:
    count[num] += 1

duplicates = []
for k in range(1, n + 1):
    if count[k] > 1:
        duplicates.extend([k] * (count[k] - 1))

missings = []
for m in range(1, n + 1):
    if count[m] == 0:
        missings.append(m)

# Check if the counts are equal
if len(duplicates) != len(missings):
    print(-1)
else:
    # Sort duplicates in descending order and missings in ascending order
    duplicates.sort(reverse=True)
    missings.sort()

    # Pair the largest duplicate with the smallest missing
    total_cost = 0
    for i in range(len(duplicates)):
        k = duplicates[i]
        m = missings[i]
        total_cost += (m - k)
    print(total_cost)