import bisect

n = int(input())
A = list(map(int, input().split()))
C = [A[i] - (i + 1) for i in range(n)]
dp = []
for c in C:
    idx = bisect.bisect_right(dp, c)
    if idx == len(dp):
        dp.append(c)
    else:
        dp[idx] = c
print(n - len(dp))