#include <cstdio>
#include <cstdlib>
using namespace std;
const int MAXN=1111; 
int N,Num[MAXN],F[MAXN][MAXN]={0};

inline int Max(int a,int b) {return a>b?a:b;}

int main()
{
	freopen("seqgame.in","r",stdin);
	freopen("seqgame.out","w",stdout);
	scanf("%d\n",&N);
	for(int i=1;i<=N;i++) scanf("%d",&Num[i]);
	for(int i=1;i<=N;i++)
	{ 
		if(Num[i]==i) F[i][0]=F[i-1][0]+1;else F[i][0]=F[i-1][0];
		for(int j=1;j<=i;j++)
		{
			F[i][j]=F[i-1][j-1];
			if(Num[i]==i-j) F[i][j]=Max(F[i][j],F[i-1][j]+1);
			else F[i][j]=Max(F[i][j],F[i-1][j]);
		}
	}
	int ans=0;
	for(int i=0;i<=N;i++) ans=Max(ans,F[N][i]);
	printf("%d\n",ans);
	return 0;
}