#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

int n, a[1010];
int dp[1010][1010], dp2[1010] = {};

int ok(int l, int r){
    int &res = dp[l][r];
    if(res != -1) return res;
    res = false;
    if(l+0 == r) res = true;
    for(int i = l+2; i < r; i += 2) if(ok(l,i) && ok(i,r)) res = true;
    if(abs(a[l] - a[r-1]) <= 1 && ok(l+1, r-1)) res = true;
    return res;
}

int main(){
    while(cin >> n && n){
        rep(i, n) cin >> a[i];
        memset(dp, -1, sizeof dp);
        memset(dp2, 0, sizeof dp2);
        dp2[0] = 0;
        rep(i, n){
            dp2[i+1] = max(dp2[i+1], dp2[i]);
            for(int j = i + 2; j <= n; j += 2){
                if(ok(i, j)) dp2[j] = max(dp2[j], dp2[i] + j - i);
            }
        }
        cout << dp2[n] << endl;
    }
}