N=int(raw_input())
A=map(int,raw_input().split())
ans = 10000
for n in range(1,N+1,2):
    plus = 0
    minus =0
    for i in range(n/2):
        if A[i] > (i+1):
            plus += A[i]-(i+1)
        else:
            minus += (i+1)-A[i]
        if A[n-1-i] > (i+1):
            plus += A[n-1-i]-(i+1)
        else:
            minus += (i+1)-A[n-1-i]
    if A[n/2]>(n/2+1):
        plus += A[n/2]-(n/2+1)
    else:
        minus += (n/2+1)-A[n/2]
    for i in range(n,N):
        plus += A[i]
    #print n, plus, minus
    if plus >= minus:
        ans=min(ans,max(plus,minus))
print ans