#include <bits/stdc++.h>

using namespace std;

int a[55];
int dp[51][51][51][51];

int main(){
    ios_base::sync_with_stdio(0);cin.tie(0);

    int n;
    cin >> n;
    for(int i = 1; i <= n; i++){
        cin >> a[i];
        for(int k = 1; k <= 50; k++)
            for(int l = k; l <= 50; l++)
                if(k <= a[i] && l >= a[i])
                    dp[i][i][k][l] = 1;
    }
    for(int dl = 2; dl <= n; dl++)
        for(int i = 1; i <= n - dl + 1; i++){
            int j = i + dl - 1;
            for(int k = 1; k <= 50; k++)
                for(int l = k; l <= 50; l++){
                    if(a[i] <= k)
                        dp[i][j][a[i]][l] = max(dp[i][j][a[i]][l],dp[i + 1][j][k][l] + 1);
                    else
                        dp[i][j][a[i]][l] = max(dp[i][j][a[i]][l],dp[i + 1][j][k][l]);
                    if(a[j] >= l)
                        dp[i][j][k][a[j]] = max(dp[i][j][k][a[j]],dp[i][j - 1][k][l] + 1);
                    else
                        dp[i][j][k][a[j]] = max(dp[i][j][k][a[j]],dp[i][j - 1][k][l]);
                    if(a[j] <= k && a[i] >= l && a[j] <= a[i])
                        dp[i][j][a[j]][a[i]] = max(dp[i][j][a[j]][a[i]],dp[i + 1][j - 1][k][l] + 2);
                }
        }
    int ans = 0;
    for(int i = 1; i <= 50; i++)
        for(int j = 1; j <= 50; j++)
            ans = max(ans,dp[1][n][i][j]);
    cout << ans;
    return 0;
}