#include <iostream>
#include <fstream>
#include <vector>

using namespace std;
const int maxn = 250;
int main() {
    ios_base::sync_with_stdio(false);
    ifstream cin("248.in");
    ofstream cout("248.out");
    int n;
    cin >> n;
    vector<int> cows(n);
    vector<vector<int>> dp(n + 1, vector<int>(n + 1, 0));

    for(int i = 0; i < n; i++) {
        cin >> cows[i];
        dp[i][i] = cows[i];
    }
    
    for(int i = n - 1; i >= 0; i--) {
        for(int j = i + 1; j < n; j++) {
            dp[i][j] = -2e9;
            
            if(i + 1 < n) {
                if(dp[i + 1][j] == cows[i]) {
                    dp[i][j] = max(dp[i][j], cows[i] + 1);
                }
            }
            if(j - 1 >= 0) {
                if(dp[i][j - 1] == cows[j]) {
                    dp[i][j] = max(dp[i][j], cows[j] + 1);
                }
            }
            
            for(int k = i + 1; k < j; k++) {
                if(dp[i][k] == dp[k + 1][j] and dp[k + 1][j] != -2e9) {
                    dp[i][j] = max(dp[i][j], dp[i][k] + 1);
                }
            }
        }
    }
    
    int result = 0;
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < n; j++) {
            result = max(result, dp[i][j]);
        }
    }
    cout << result << endl;
    
    return 0;
}