#include <cstdio>
#include <algorithm>
using namespace std;
const int N = 1000 + 10, INF = 0x7ffffff;
int n, a[N], f[N][N], s;
int main() {
    freopen("seqgame.in", "r", stdin);
    freopen("seqgame.out", "w", stdout);
    scanf("%d", &n);
    for(int i=1; i<=n; i++)
        scanf("%d", a+i);
    for(int i=1; i<=n; i++) {
        f[i][0] = f[i-1][0] + (a[i] == i);
        for(int j=1; j<i; j++)
            f[i][j] = max(f[i-1][j] + (a[i] == i - j), f[i-1][j-1]);
    }
    s = *max_element(f[n]+1, f[n]+n+1);
    printf("%d\n", s);
    return 0;
}