#include <iostream>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <queue>
#include <map>
#include <cstring>
#include <iomanip>
#include <vector>
#include <stack>
#define LL long long
using namespace std;
const int N = 1e3 + 5;
const int M = 1e6 + 5;
const int INF = 0x3f3f3f3f;
int f[N][N];
int main()
{
    int n, ans = 0;
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)
	{
        scanf("%d", f[i] + i);
        ans = max(ans, f[i][i]);
    }
    for(int i = 2; i <= n; i++)
	{ 
        for(int l = 1; l + i -1 <= n; l++)
		{
            int r = l + i - 1;
            for(int k = l; k < r; k++)
            {
                if(f[l][k] == f[k + 1][r] && f[l][k])
				{
                    f[l][r] = max(f[l][r], f[l][k] + 1);
                    ans = max(ans, f[l][r]);
                }
			}
        }
	}
    cout << ans;		
    return 0;
}