N = int(input())

A = list(map(int, input().split())) + [0] * N

a = sum(A)

ans = a - 1
for i in range(1, N):
    if (i + 1) ** 2 > a:
        break
    
    ans = min(ans, sum([A[j] - j - 1 if A[j] > j else 0 for j in range(i)]) + sum([A[j] - 2 * i - 1 + j if A[j] > 2 * i - j else 0 for j in range(i, 2 * i + 1)]) + sum([A[j] for j in range(2 * i + 1, 2 * N)]))

    
print(ans)