import bisect

n = int(input())
a = list(map(int, input().split()))
b = [a[i] - (i + 1) for i in range(n)]

tails = []
for num in b:
    idx = bisect.bisect_right(tails, num)
    if idx == len(tails):
        tails.append(num)
    else:
        tails[idx] = num

print(n - len(tails))