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

#define fs first
#define sc second

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; }


// std::vector Declaration
template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts) {
    return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

// std::vector Initialization
template<typename T>
void fill_all(T& arr, const T& v) { arr = v; }
template<typename T, typename U>
void fill_all(T& arr, const U& v) { for (auto& i : arr) fill_all(i, v); }

// std::vector Debug
template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v ) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::pair Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    os << "(" << p.first << " " << p.second << ")";
    return os;
}

// std::set Debug
template<typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::multiset Debug
template<typename T>
ostream& operator<<(ostream& os, const multiset<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::map Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const map<T, U>& mp) {
    os << "{";
    bool a = 1;
    for (auto e : mp) {
        os << (a ? "" : " ");
        os << e.first << ":" << e.second;
        a = 0;
    }
    os << "}";
    return os;
}

// std::tuple Debug
template<int N, class Tuple>
void out(ostream& os, const Tuple& t){}
template<int N, class Tuple, class H, class ...Ts>
void out(ostream& os, const Tuple& t) {
    if (N) os << " ";
    os << get<N>(t);
    out<N+1,Tuple,Ts...>(os, t);
}
template<class ...Ts>
ostream& operator<<(ostream& os, const tuple<Ts...>& t) {
    os << "(";
    out<0,tuple<Ts...>,Ts...>(os, t);
    os << ")";
    return os;
}

// Debug
#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl


struct edge { int to, cost; };
ostream& operator<<(ostream& os, const edge& e) {
    os << "(" << e.to << ", " << e.cost << ")";
    return os;
}

const LL LINF = 1LL<<60;
const int IINF = 1<<30;
const LL MOD = 1e9+7;

int main()
{
    vector<int> ns;
    vector<vector<int>> ws;

    while (true) {
        int n; cin >> n;
        if (n == 0) {
            break;
        }
        vector<int> w(n);
        for (int i = 0; i < n; ++i) {
            cin >> w[i];
        }
        ns.push_back(n);
        ws.push_back(w);
    }

    for (int t = 0; t < ns.size(); ++t) {
        int n = ns[t];
        auto w = ws[t];

        auto dp = make_v<int>(n, n+1);
        fill_all(dp, -IINF);

        for (int i = 0; i < n; ++i) {
            dp[i][i] = 0;
        }

        for (int k = 1; k <= n; ++k) {
            for (int i = 0; i+k <= n; ++i) {
                for (int j = i+1; j < i+k; ++j) {
                    chmax(dp[i][i+k], dp[i][j] + dp[j][i+k]);
                }
                if (k >= 2 && abs(w[i] - w[i+k-1]) <= 1) {
                    chmax(dp[i][i+k], dp[i+1][i+k-1] + 2);
                }
            }
        }

        vector<int> dp2(n+1, -IINF);
        dp2[0] = 0;
        for (int i = 0; i < n; ++i) {
            chmax(dp2[i+1], dp2[i]);
            for (int j = i+1; j <= n; ++j) {
                chmax(dp2[j], dp2[i] + dp[i][j]);
            }
        }

        cout << dp2[n] << endl;
    }

    return 0;
}



Measured memory_bytes: 3212