import bisect

def main():
    import sys
    input = sys.stdin.read().split()
    N = int(input[0])
    A = list(map(int, input[1:N+1]))
    
    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))

if __name__ == "__main__":
    main()