Go: package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strconv"
)

const (
	initialBufSize = 10000000
	maxBufSize     = 100000000
)

var sc *bufio.Scanner

func initScanner(r io.Reader) *bufio.Scanner {
	buf := make([]byte, initialBufSize)

	sc := bufio.NewScanner(r)
	sc.Buffer(buf, maxBufSize)
	sc.Split(bufio.ScanWords) // bufio.ScanLines
	return sc
}

func main() {
	sc = initScanner(os.Stdin)
	resolve()
}

func nextInt(sc *bufio.Scanner) int {
	sc.Scan()
	a, _ := strconv.Atoi(sc.Text())
	return int(a)
}
func nextString(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextIntSlice(sc *bufio.Scanner, n int) (a []int) {
	a = make([]int, n)
	for i := 0; i < n; i++ {
		a[i] = nextInt(sc)
	}
	return a
}

var dp [][]int

func initDP(n int) {
	dp = make([][]int, n+1)
	for i := 0; i < len(dp); i++ {
		dp[i] = make([]int, n+1)
		for j := 0; j < len(dp[0]); j++ {
			dp[i][j] = -1
		}
	}
}

func resolve() {
	ans := []int{}
	for true {
		n := nextInt(sc)
		if n == 0 {
			break
		}
		as := nextIntSlice(sc, n)
		initDP(n)
		_rec(as, n, 0, n)
		ans = append(ans, dp[0][n])
	}
	for _, v := range ans {
		fmt.Println(v)
	}
	return
}

func _rec(as []int, n, l, r int) int {
	if dp[l][r] > -1 {
		return dp[l][r]
	}
	if r-l == 1 { // prevent to hit daruma
		dp[l][r] = 0
		return dp[l][r]
	}
	if r-l == 2 {
		if abs(as[l]-as[r-1]) <= 1 {
			dp[l][r] = 2
		} else {
			dp[l][r] = 0
		}
		return dp[l][r]
	}
	if abs(as[l]-as[r-1]) <= 1 {
		v := _rec(as, n, l+1, r-1)
		// if we can remove internal blocks
		if (r - 1 - (l + 1)) == v {
			dp[l][r] = v + 2
			return dp[l][r]
		} else {
			dp[l][r] = v
		}
	}
	for i := l + 1; i < r; i++ {
		v := _rec(as, n, l, i) + _rec(as, n, i, r)
		if dp[l][r] < v {
			dp[l][r] = v
		}
	}
	return dp[l][r]
}

func abs(v int) int {
	if v < 0 {
		return -v
	}
	return v
}

C++: #include <bits/stdc++.h>

#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT_ON(bit, i) (bit & (1LL << i))

typedef long long LL;
template<typename T> using V = std::vector<T>;
template<typename T> using VV = std::vector<std::vector<T>>;
template<typename T> using VVV = std::vector<std::vector<std::vector<T>>>;
template<typename T> using VVVV = std::vector<std::vector<std::vector<std::vector<T>>>>;

template<class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
template<class T> inline void print(T x) { std::cout << x << std::endl; }
template<class T> inline void print_vec(const std::vector<T> &v) { for (int i = 0; i < v.size(); ++i) {  if (i != 0) {std::cout << " ";} std::cout << v[i];} std::cout << "\n"; }
template<class T> inline bool inside(T y, T x, T H, T W) {return 0 <= y and y < H and 0 <= x and x < W; }
template<class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) { return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)); }
template<class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }
template<class T> inline std::vector<T> unique(std::vector<T> v) {
    sort(v.begin(), v.end());
    v.erase(unique(v.begin(), v.end()), v.end());
    return v;
}

const int INF = 1 << 30;
const double EPS = 1e-9;
const std::string YES = "YES", Yes = "Yes", NO = "NO", No = "No";
const std::vector<int> dy4 = { 0, 1, 0, -1 }, dx4 = { 1, 0, -1, 0 };    // 4近傍（右, 下, 左, 上）
const std::vector<int> dy8 = { 0, -1, 0, 1, 1, -1, -1, 1 }, dx8 = { 1, 0, -1, 0, 1, 1, -1, -1 };

using namespace std;


int solve(V<int> &A) {
    int N = A.size();
    VV<int> dp(N + 1, V<int>(N + 1));  // 閉区間

    FOR(w, 2, N + 1) {
        FOR(left, 0, N) {
            int right = left + w - 1;
            if (right >= N) {
                continue;
            }

            // left + 1からright - 1までが取り除けて、left, rightの組で取り除ける
            if (dp[left + 1][right - 1] == w - 2 and abs(A[left] - A[right]) <= 1) {
                dp[left][right] = w;
            }
            // leftからrightの最大値
            FOR(middle, left, right) {
                dp[left][right] = max(dp[left][right], dp[left][middle] + dp[middle + 1][right]);
            }
        }
    }

    return dp[0][N - 1];
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    while (true) {
        int N;
        cin >> N;
        if (N == 0) {
            break;
        }

        V<int> A(N);
        FOR(i, 0, N) {
            cin >> A[i];
        }
        print(solve(A));
    }

    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vs = vector<string>;
using msi = map<string, int>;
using mii = map<int, int>;
using psi = pair<string, int>;
using pii = pair<int, int>;
using vlai = valarray<int>;
#define rep(i,n) for(int i=0;i<n;i++)
#define range(i,s,n) for(int i=s;i<n;i++)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define fs first
#define sc second
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define INF (ll)1E9
#define EPS 1E-9
#define MOD 1000000007
#define PI 3.1415926535897932384

template <class T>ostream &operator<<(std::ostream &o, const vector<T> &v)
{
	rep(i, v.size()) {
		o << (i > 0 ? " " : "") << v[i];
	}
	return o;
}

//int dx[]={1,1,1,0,-1,-1,-1,0},dy[8]={1,0,-1,-1,-1,0,1,1,1};

//constexpr ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }

void solve(int n) {
	vi v(n, 0);
	vector<vector<bool>> dp(n + 1, vector<bool>(n, false));
	rep(i, n) {
		cin >> v[i];
	}
	for (int l = 2; l <= n; l += 2) {
		for (int i = 0; i + l <= n; i++) {
			bool b = false;
			if (l != 2) {
				for (int j = 2; j<l; j += 2) {
					if (dp[j][i] && dp[l - j][i + j])
						b = true;
				}
			}
			int dif = abs(v[i] - v[i + l - 1]);
			if (dif <= 1&&(l==2?1:dp[l-2][i+1]))b = true;
			if (b)dp[l][i] = true;
			//cout << l << " " << i << " " << dp[l][i] << endl;
		}
	}
	int pos = 0;
	queue<pii> q;
	q.push({ 0,0 });
	while (!q.empty()) {
		pii p = q.front(); q.pop();
		if (p.fs == n) {
			cout << n-p.sc << endl;
			return;
		}
		for (int i = 2; i < n + 1;i+=2) {
			if (dp[i][p.fs]) {
				q.push({ p.fs + i,p.sc });
			}
		}
		q.push({ p.fs+1 ,p.sc + 1 });
	}

}

int main(void) {
	int n;
	while (cin >> n, n) {
		solve(n);
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <map>
#include <cmath>
#include <set>
#include <algorithm>

typedef long long ll;
const ll MOD = 1e9 + 7;
const long long INF = 1LL<<60;
const double PI=3.14159265358979323846;
const int NMAX=100005;
using namespace std;


vector<ll> a(6001);
ll dp[6001][6001];


void solve(int n){
    vector<ll> w(n);
    for(int i = 0; i < n; i++){
        cin >> w[i];
    }
    ll dp[n+1][n+1];
    for(int i = 0; i <= n; i++){
        for(int j = 0; j <= n; j++){
            dp[i][j] = 0;
        }
    }

    for(int len = 1; len <= n; len++){
        for(int l = 0; l < n; l++){
            int r = l+len;
            if(r > n) break;
            if(r-1 != l && dp[l+1][r-1] == len-2 &&abs(w[r-1]-w[l]) <= 1){
                dp[l][r] = dp[l+1][r-1]+2;
            }
            for(int mid = l; mid <= r; mid++){
                dp[l][r] = max(dp[l][r],dp[l][mid]+dp[mid][r]);
            }
        }
    }

    cout << dp[0][n] << endl;
}

int main(){
    int n;
    while(true){
        cin >> n;
        if(n == 0) break;
        solve(n);
    }
    return 0;
}