#include<iostream>
#include<cstdio>
#include<cstdlib>
using namespace std;
int n,w[1005]={0};
int q[1005][1005]={0};
int main()
{
	freopen ("seqgame.in","r",stdin);
	freopen ("seqgame.out","w",stdout);
	cin>>n;
	for (int i=1;i<=n;i++)
	{
		cin>>w[i];
		if (w[i]==i)
		{
			q[i][0]=q[i-1][0]+1;
		}
		else
		{
			q[i][0]=q[i-1][0];
		}
	}
	for (int i=1;i<=n;i++)
	{
		for (int j=0;j<=i;j++)
		{
			q[i][j]=q[i-1][j];
			if (w[i]==i-j)
			{
				q[i][j]=max(q[i][j],q[i-1][j]+1);
			}
			q[i][j]=max(q[i][j],q[i-1][j-1]);
		}
	}
	int answer=0;
	for (int i=0;i<=n;i++)
	{
		if (q[n][i]>answer)
			answer=q[n][i];
	}
	cout<<answer;
	return 0;
}