#include<bits/stdc++.h>

#define maxn 4050

#define maxn2 1000005

using namespace std;

int dp[maxn][maxn],store[maxn];

int maxx[maxn2];

int main(){

    int n,ans = ;

    scanf("%d",&n);

    for(int i = ;i <= n;++i){

        scanf("%d",&store[i]);

    }

    dp[][] = ;

    for(int i = ;i <= n;++i){

        int k = -;

        for(int j = ;j < i;++j){

            if(k == -){

                if(j > ) dp[i][j] = ;

                else dp[i][j] = ;

            }

            else{

                dp[i][j] = max(dp[i][j],dp[j][k] + );

            }

            if(j >  && store[i] == store[j]) k = j;

            ans = max(ans,dp[i][j]);

        }

    }

    printf("%d\n",ans);

    return ;

}

/*

10

1 3 2 4 5 2 6 2 5 6

*/