import bisect

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

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

print(n - len(dp))