n = int(input())
A = list(map(int,input().split()))+[0]*n
ans = 10**10

for i in range(n):
    
    count = 0
    low = 0
    high = 0
    for j in range(i+1):
        dif = A[j]-j-1
        if dif < 0:
            low -= dif
        else:
            high += dif

        count += dif
    
    for j in range(i+1,2*i+1):
        
        dif = A[j]-(2*i+1-j)
        if dif < 0:
            low -= dif
        else:
            high += dif

        count += dif

    for j in range(2*i+1,n):
        high += A[j]
        count += A[j]

    move = max(high,low)

    if count >= 0:
        ans = min(ans,move)
print(ans)