#include <bits/stdc++.h>
#include <cstdlib>

using namespace std;

int solve(int i, int j, vector<int> &ducks, vector< vector<int> > &dp) {
    if (dp[i][j] == -1) {
        if (i+1 > j) dp[i][j] = 0;
        else if (ducks[i] == ducks[j]) {
            dp[i][j] = 1 + solve(i+1, j-1, ducks, dp);
        } else {
            dp[i][j] = max(solve(i+1, j, ducks, dp), solve(i, j-1, ducks, dp));
        }
    }
    return dp[i][j];
}
int main(int argc, char** argv) {
    ios::sync_with_stdio(false);
    int N;
    while(cin >> N) {
        vector< vector<int> > dp(N, vector<int>(N, -1));
        vector<int> ducks(N);
        for(int i = 0; i < N; ++i) cin >> ducks[i];
        cout << solve(0, N-1, ducks, dp) << endl;
        
    }
    return 0;
}