#include<iostream>
#include<cstdio>
//#include<algorithm>
#define re register
using namespace std;
int read(){
	int x=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch='-')f=-1;ch=getchar();}
	while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
	return x*f;
} 
int n,a[1110],dp[1110][1110],ans;
inline int max(const int &a,const int &b){return(a>b?a:b);}
int main(){
	n=read();
	for(int i=1;i<=n;i++)a[i]=read();
	for(re int i=1;i<=n;i++){
		for(re int j=0;j<=i;j++){
			if(j>0)dp[i][j]=dp[i-1][j-1];
			dp[i][j]=max(dp[i][j],dp[i-1][j]+(a[i]==i-j));
			ans=max(ans,dp[i][j]);
		}
	}
	printf("%d",ans);
	return 0;
}

RT,用scanf读就可以AC，写了这个快读反而WA了一个点。为啥？？？