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

cnt = {}
tot = 0
for i, a in enumerate(A):
    x = a - i
    cnt[x] = cnt.get(x, 0) + 1
    tot += cnt.get(x + 1, 0)
    cnt[x] += cnt.get(x + 1, 0)
    cnt[x + 1] = 0
    ans[i] += tot

A = [-a for a in A[::-1]]
ans = ans[::-1]

cnt = {}
tot = 0
for i, a in enumerate(A):
    x = a - i
    cnt[x] = cnt.get(x, 0) + 1
    tot += cnt.get(x + 1, 0)
    cnt[x] += cnt.get(x + 1, 0)
    cnt[x + 1] = 0
    ans[i] += tot

print(min(ans))