#include <bits/stdc++.h>
using namespace std;

//using Graph = vector<vector<edge>>;
using dou =long double;
string yes="yes";
string Yes="Yes";
string YES="YES";
string no="no";
string No="No";
string NO="NO";

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
#define rep(i, n)         for(int i = 0; i < (int)(n); i++)
#define brep(n)           for(int bit=0;bit<(1<<n);bit++)
#define erep(i,container) for (auto i : container)
#define irep(i, n)        for(int i = n-1; i >= (int)0; i--)
#define rrep(i,m,n) for(ll i = m; i < (ll)(n); i++)
#define reprep(i,j,h,w) rep(i,h)rep(j,w)
#define all(x) (x).begin(),(x).end()
#define VEC(type,name,n) std::vector<type> name(n);rep(i,n)std::cin >> name[i];
#define pb push_back
#define pf push_front
#define query int qq;std::cin >> qq;rep(qqq,qq)
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define itn int
#define mp make_pair
#define sum(a) accumulate(all(a),0ll)
#define keta fixed<<setprecision
#define vvector(name,typ,m,n,a)vector<vector<typ> > name(m,vector<typ> (n,a))
#define vvvector(name,t,l,m,n,a) vector<vector<vector<t> > > name(l, vector<vector<t> >(m, vector<t>(n,a)));
#define vvvvector(name,t,k,l,m,n,a) vector<vector<vector<vector<t> > > > name(k,vector<vector<vector<t> > >(l, vector<vector<t> >(m, vector<t>(n,a)) ));
#define case std::cout <<"Case #" <<qqq+1<<": "
#define res resize
#define as assign
#define ffor for(;;)
#define ppri(a,b) std::cout << a<<" "<<b << std::endl
#define pppri(a,b,c) std::cout << a<<" "<<b <<" "<< c<<std::endl
#define aall(x,n) (x).begin(),(x).begin()+(n)
#define ssum(a) accumulate(a,0ll) 

typedef long long ll;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;
//typedef long long T;
const int INF = 2000000000;
const ll INF64 = 922337203685477580ll;
const ll MOD = 1000000007ll;
const dou pi=3.141592653589793;
int main(){
    ffor{
        int n;
        std::cin >> n;
        if(n==0)break;
        
        VEC(int,a,n);
        
        
        vvector(dp,int,n+1,n+1,0);
        for (int ii = 0; ii < n+1; ii++) {
            rep(i,n+1-ii){
                for(int jj=0;jj<=ii;jj++){
                    chmax(dp[i][ii+i],dp[i][i+jj]+dp[i+jj][ii+i]);
                    
                }
                if(i!=0&&i+ii!=n){
                    if(dp[i][i+ii]==ii&&abs(a[i-1]-a[i+ii])<=1)chmax(dp[i-1][i+ii+1],dp[i][i+ii]+2);
                }
            }
        }
        int ans=0;
        reprep(i,j,n+1,n+1){
           // pppri(i,j,dp[i][j]);
            chmax(ans,dp[i][j]);
        }
        std::cout << ans << std::endl;
        
        
        /*
        vvector(dp,bool,n+1,n+1,0);
        rep(i,n+1)dp[i][i]=1;
        
        for (int ii = 0; ii < n+1; ii+=2) {
            rep(i,n+1-ii){
                for(int jj=0;jj<=ii;jj+=2){
                    if(dp[i][i+jj]&&dp[i+jj][ii+i])dp[i][ii+i]=1;
                    
                }
                if(i!=0&&i+ii!=n){
                        if(dp[i][i+ii]&&abs(a[i-1]-a[i+ii])<=1)dp[i-1][i+ii+1]=1;
                }
            }
        }
        */
    }
    
}


Measured memory_bytes: 3336