N = int(input())

A = list(map(int, input().split()))

ans = 10 ** 9

for i in range((N + 1) // 2):
    
    for j in range(N - 2 * i):
        x = sum(A[:j]) + sum(A[j + 2 * i + 1:])
        y = 0
        for k in range(i):
            
            if A[k + j] > k:
                
                x += A[k + j] - k - 1
                
            else:
                y += j + 1 - A[k + j]
                
        for k in range(i + 1):
            
            if A[j + i + k] > (i + 1) - k:
                
                x += A[j + i + k] - i - 1 + k
                
            else:
                
                y += i + 1 - k - A[j + i + k]
                
        if x >= y:
            
            ans = min(ans, x)
            
print(ans)