#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i, n) for(int i = 0; i < n; i++)
#define REPR(i, n) for(int i = n; i >= 0; i--)
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define FORR(i, m, n) for(int i = m; i >= n; i--)
#define INF (ll)2e9
#define MOD ((ll)1e9+7)
#define ALL(v) v.begin(), v.end()
#define SZ(x) ((int)(x).size())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
#define next asdnext
#define prev asdprev
#define bit(n) (1LL<<(n))
#define cauto const auto&
#define println(v) cout << v << "\n";

void show(vector<vector<ll>>& arr, int w) {
    cout << right << setw(w) << "#";
    REP(i, SZ(arr[0]))  {
        cout << right << setw(w) << i;
    }
    cout << endl;
    REP(i, SZ(arr)) {
        cout << right << setw(w) << i;
        REP(j, SZ(arr[0])){
            cout << right << setw(w) << arr[i][j];
        }
        cout << endl;
    }
}

inline vector<vector<vector<ll>>> makeVector(ll i, ll j, ll k) {
    vector<vector<vector<ll>>> v(i, vector<vector<ll>>(j, vector<ll>(k, 0)));
    return v;
}
inline vector<vector<ll>> makeVector(ll i, ll j) {
    vector<vector<ll>> v(i, vector<ll>(j, 0));
    return v;
}

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template <class InputIterator>
InputIterator adv(InputIterator x, typename std::iterator_traits<InputIterator>::difference_type n) {
    advance(x, n);
    return x;
}

class mod {
    static ll fact[];
public:

    template<class... A>
    static ll mul(A... args) {
        ll res = 1;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = (res * i) % MOD;
        }
        return res;
    }

    static ll power(ll base, ll exp) {
        if (exp == 0) return 1;
        if (exp & 1) {
            return mul(base, power(base, exp - 1));
        } else {
            ll p = power(base, exp / 2);
            return mul(p, p);
        }
    }

    static ll factorial(int n) {
        if (fact[n] != 0) return fact[n];
        if (n == 0) return 1;
        return fact[n] = mul(n, factorial(n - 1));
    }

    static ll inverse(int n) {
        return power(n, MOD - 2);
    }

    static ll comb(int n, int r) {
        if (r < 0 || r > n) return 0;
        return mul(factorial(n), inverse(factorial(n - r)), inverse(factorial(r)));
    }

    template<class... A>
    static ll div(ll dividend, A... args) {
        ll res = dividend;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = mul(res, inverse(i));
        }
        return res;
    }

    template<class... A>
    static ll add(A... args) {
        ll res = 0;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = (res + i) % MOD;
        }
        return res;
    }

    template<class... A>
    static ll sub(ll l, A... args) {
        ll res = l;
        for (ll i : std::initializer_list<ll>{args...}) {
            res = (res - i + MOD) % MOD;
        }
        return res;
    }
};
ll mod::fact[(int) 1e6 + 1];

int ni() {
    int i;
    cin >> i;
    return i;
}
ll nll() {
    ll l;
    cin >> l;
    return l;
}

int main()
{
    while (true) {
        int N = ni();

        if (N == 0) break;

        vector<ll> w(N);
        REP(i, N) cin >> w[i];

        auto dp = makeVector(N + 1, N + 1);

        FOR(len, 2, N + 1) {
            REP(i, N - len + 1) {

                FOR(j, i+1, i+len) {
                    dp[i][i + len] = max(dp[i][i+len], dp[i][j] +dp[j][i+len]);
                }

                if (dp[i + 1][i + len - 1] == len - 2 && abs(w[i] - w[i + len - 1]) <= 1) {
                    dp[i][i+len] = len;
                }
                dp[i][i + len] = max(dp[i][i + len],
                                     dp[i][i + len - 2] + (abs(w[i + len - 2] - w[i + len - 1]) <= 1 ? 2 : 0));
                dp[i][i + len] = max(dp[i][i + len], dp[i + 2][i + len] + (abs(w[i] - w[i + 1]) <= 1 ? 2 : 0));
            }
        }

        //show(dp, 4);

        println(dp[0][N]);
    }

    return 0;
}


Measured memory_bytes: 3636