#include <iostream>
#include <algorithm>
#include <cstring>
#define MAXN 55

using namespace std;

int n;
int arr[MAXN];
int dp[MAXN][MAXN][MAXN][MAXN];

int main() {
    freopen("subrev.in", "r", stdin);
    freopen("subrev.out", "w", stdout);
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> n;
    for (int i = 1; i <= n; i++)
        cin >> arr[i];
    for (int i = 1; i <= n; i++) {
        for (int a = 1; a <= arr[i]; a++) {
            for (int b = arr[i]; b <= 50; b++)
                dp[i][i][a][b] = 1;
        }
    }
    for(int i = 1; i <= n; ++i) {
        for(int j = 1; j <= n; ++j) {
            for(int k = 0; k <= 50; ++k) {
                for(int m = 0; m <= 50; ++m) {
                    if(k > m) dp[i][j][k][m] = -1000;
                }
            }
        }
    }

    for (int i = n; i >= 1; i--) {
        for (int j = i+1; j <= n; j++) {
            for (int a = 50; a >= 1; a--) {
                for (int b = a; b <= 50; b++) {
                    dp[i][j][a][b] = max(dp[i+1][j][a][b], dp[i][j-1][a][b]);
                    bool lim1 = (arr[i] >= a && arr[i] <= b), lim2 = (arr[j] >= a && arr[j] <= b);
                    if (lim1)
                        dp[i][j][a][b] = max(dp[i][j][a][b], 1 + dp[i+1][j][arr[i]][b]);
                    if (lim2)
                        dp[i][j][a][b] = max(dp[i][j][a][b], 1 + dp[i][j-1][a][arr[j]]);
                    if (lim1 && lim2)
                        dp[i][j][a][b] = max(dp[i][j][a][b], 2 + dp[i+1][j-1][min(arr[i], arr[j])][max(arr[i], arr[j])]);

                    //cases where we reverse
                    if(arr[j] >= a)
                        dp[i][j][a][b] = max(dp[i][j][a][b], 1 + dp[i+1][j-1][arr[j]][b]);
                    if(arr[i] <= b)
                        dp[i][j][a][b] = max(dp[i][j][a][b], 1 + dp[i+1][j-1][a][arr[i]]);
                }
            }
        }
    }
    /*for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            cout << dp[i][j][1][50] << " ";
        }
        cout << "\n";
    }*/
    cout << dp[1][n][1][50];
}