#include <fstream>
using namespace std;
int max(int x,int y)
{
	if(x>y)return x;
	else return y;
}
int main()
{
	int n,a[1001],d=0,f[1001][1001],i,j;
	ifstream in("seqgame.in");
	ofstream out("seqgame.out");
	in>>n;
	for(i=1;i<=n;i++)in>>a[i];
	for(i=1;i<=n;i++)
	    for(j=1;j<=i;j++)
	if(a[i]==j)
	{
		f[i][j]=max(f[i-1][j-1]+1,f[i-1][j]);
		d=max(f[i][j],d);
	}
	else f[i][j]=max(f[i-1][j-1],f[i-1][j]);
    out<<d;	
	in.close();
	out.close();
	return 0;
}