题解，AC的

#include <cstdio>
using namespace std;

const int max_n = 50;

int dp[max_n][max_n][max_n+1][max_n+1] = {}, a[max_n];

void upd(int& a, int b) { a = ((a > b)? a:b); }

int main()
{
	int n;
	
	scanf("%d", &n);
	for (int i = 0; i < n; i++)
		scanf("%d", a + i);
	
	for (int i = 0; i < n; i++)
		for (int j = 1; j <= a[i]; j++)
			for (int k = a[i]; k <= max_n; k++)
				dp[i][i][j][k] = 1;
	
	for (int l1 = 2; l1 <= n; l1++)
		for (int l = 0, r = l1 - 1; r < n; l++, r++)
			for (int l2 = 1; l2 <= max_n; l2++)
				for (int lp = 1, rp = l2; rp <= max_n; lp++, rp++)
				{
					if (lp != max_n)
						upd(dp[l][r][lp][rp], dp[l][r][lp+1][rp]);
					upd(dp[l][r][lp][rp], dp[l][r][lp][rp-1]);
					
					upd(dp[l][r][lp][rp], dp[l+1][r][lp][rp] + (lp == a[l]));
					upd(dp[l][r][lp][rp], dp[l][r-1][lp][rp] + (rp == a[r]));
					
					upd(dp[l][r][lp][rp], dp[l+1][r-1][lp][rp] + (lp == a[r]) + (rp == a[l]));
				}
	
	printf("%d\n", dp[0][n-1][1][max_n]);
	
	return 0;
}