N = input()
A = map(int, raw_input().split())

pyramid = range(1, (N/2)+1) + range((N/2)+1, 0, -1)
#print A, pyramid

dif = [ A[i]-pyramid[i] for i in xrange(N)]
#print dif

pos = sum( [d for d in dif if d > 0] )
neg = sum( [d for d in dif if d < 0] ) * (-1)

cnt = min(pos, neg) + abs(pos - neg)
print cnt