#include <iostream>
#include <algorithm>

using namespace std;

int dp[5000][5000];
int table[5000];

int main()
{
    int n;
    while (cin>>n)
    {
        for (int i=0; i<n; ++i)
            cin>>table[i];
        for (int i=0; i<n; ++i)
            for (int j=0; j<n; ++j)
                dp[i][j]=0;
        for (int i=0; i+1<n; ++i)
            if (table[i] == table[i+1])
                dp[i][i+1]=1;
        for (int diff=2; diff<n; ++diff)
            for (int i=0; i+diff<n; ++i)
            {
                int l = dp[i][i+diff-1];
                int r = dp[i+1][i+diff];
                int c = dp[i+1][i+diff-1] + ((table[i] == table[i+diff])?1:0);
                dp[i][i+diff] = max(c, max(l, r));
            }
        cout<<dp[0][n-1]<<endl;
    }
}