N = int(input())
A = list(map(int, input().split()))
height = 1
ans = 190000190129219
while height * height <= sum(A):
    targ = [min(i + 1, 2 * height - 1 - i) for i in range(2 * height - 1)]
    sute = 0
    for i in range(2 * height - 1):
        if i < N:
            a = A[i]
        else:
            a = 0
        if a > targ[i]:
            sute += a - targ[i]
    for i in range(2 * height - 1, N):
        sute += A[i]
    ans = min(ans, sute)
    height += 1
print(ans)