N = int(input())
A = [0] * 100
for i, a in enumerate(map(int, input().split())):
    A[i] = a
ans = float('inf')
for i in range(1, int(sum(A)**0.5) * 2, 2):
    t = 0
    for j in range(i//2 + 1):
        t += max(A[j] - j - 1, 0)
        if j != i - j - 1:
            t += max(A[i-j-1] - j - 1, 0)
    t += sum(A[i:])
    ans = min(ans, t)
print(ans)