C++ :

#include <cstdio>
 
#define MAXN 1005UL
 
#define Max(a,b) ((a)>(b)?(a):(b))
 
using namespace std;
 
int n;
int a[MAXN];
int f[MAXN];
 
int main(){
	int i,j;
	int ans=0;
	scanf("%d",&n);
	for(i=1;i<=n;++i)
		scanf("%d",&a[i]);
	for(i=1;i<=n;++i){
		if(a[i]>i)
			continue;
		f[i]=1;
		for(j=1;j<=i;++j){
			if(f[j]+1>f[i]&&i-j>=a[i]-a[j]&&a[i]>a[j])
				f[i]=f[j]+1;
		}
		ans=Max(ans,f[i]);
	}
	printf("%d",ans);	
}