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>
using namespace std;
#define FOR(i,k,n) for(int i = (int)(k); i < (int)(n); i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) a.begin(), a.end()
#define MS(m,v) memset(m,v,sizeof(m))
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<int, int> pii;
const int MOD = 1e9 + 7;
template<class T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<class T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template<class T>
istream& operator >> (istream& is, vector<T>& v)
{
	for (auto &i : v) is >> i;
	return is;
}
template<class T>
ostream& operator<<(ostream& os, vector<T>& v)
{
	const string delimiter = "\n";
	REP(i, v.size())
	{
		os << v[i];
		if (i != v.size() - 1) os << delimiter;
	}
	return os;
}
/*--------------------template--------------------*/

int n;
vi w;
int dp[333][333];
vector<vector<bool>> del;

int del_all(int left, int right)
{
	if (dp[left][right] >= 0) return dp[left][right];
	int res = 0;
	if (left > right) res = 0;
	else if (left == right) res = 1;
	else
	{
		FOR(i, left + 1, right)
		{
			if (del_all(left, i) && del_all(i, right)) res = 1;
			if (abs(w[left] - w[right - 1]) <= 1 && del_all(left + 1, right - 1)) res = 1;
		}
	}
	return dp[left][right] = res;
}

int dp2[333];

int solve(int p)
{
	if (dp2[p] >= 0) return dp2[p];
	if (p == n) return 0;
	int res = 0;
	chmax(res, solve(p + 1));
	FOR(i, 1, n + 1)
	{
		if (del[p][i])
		{
			chmax(res, i - p + solve(i));
		}
	}
	return dp2[p] = res;
}

int main()
{
	cin.sync_with_stdio(false); cout << fixed << setprecision(10);
	while (cin >> n, n)
	{
		MS(dp, -1);
		MS(dp2, -1);
		w.resize(n);
		REP(i, n) cin >> w[i];
		int ans = 0;
		del.clear();
		del.resize(n + 1, vector<bool>(n + 1));
		REP(i, n + 1)REP(j, i)
		{
			if (del_all(j, i))
			{
				del[j][i] = true;
			}
		}
		cout << solve(0) << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#define rep(i,n) for(int i=0; i<(n); i++)
#define X first
#define Y second

using namespace std;
typedef pair<int,int> pii;

int main(){
	int n, w[1004];
	while(cin >> n, n){

		rep(i,n) cin >> w[i];

		bool dp1[305][305]={};
		vector<pii> v;
		rep(i,n-1) if( abs(w[i] - w[i+1]) < 2 ){
			dp1[i][i+1] = true;
			v.emplace_back(i, i+1);
		}
		rep(k,n){
			rep(i,n){
				if( i+k >= n ) break;
				if( dp1[i][i+k] && 0 < i && i+k+1 < n ){
					if( abs(w[i-1] - w[i+k+1]) < 2 ){
						dp1[i-1][i+k+1] = true;
						v.emplace_back(i-1, i+k+1);
					}
				}
			}
			rep(j,n) rep(i,n){
				if( i+k+j >= n ) break;
				if( dp1[i][i+k] && dp1[i+k+1][i+k+j] ){
					dp1[i][i+k+j] = true;
					v.emplace_back(i, i+k+j);
				}
			}
		}

		//for(auto t: v) cout << t.X << "," << t.Y << " "; cout << endl;
		sort( v.begin(), v.end() );

		int dp[900005] = {}, pos=0;
		rep(i,900002){
			while( pos < v.size() && v[pos].first == i ){
				int d  = v[pos].X;
				int nx = v[pos].Y + 1;
				dp[nx] = max( dp[nx], dp[i] + nx - d );
				pos++;
			}
			dp[i+1] = max( dp[i+1], dp[i] );
		}
		cout << dp[900002] << endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <algorithm>
#include <vector>
#include <iostream>
#include <string>
#include <string.h>
using namespace std;
#define pb push_back
#define mp make_pair
bool used[305][305];
int n,w[305],dp[305];
int main(){
	while(1){
		cin >> n; if(n==0) return 0;
		for(int i=1;i<=n;i++) cin>>w[i];
		memset(used,0,sizeof(used));
		for(int i=1;i<=n-1;i++){
			if(abs(w[i]-w[i+1]) <= 1) used[i][i+1] = 1;
		}
		for(int a=4;a<=n;a+=2){
			for(int i=1;i<=n+1-a;i++){
				//used[i][i+a-1]
				for(int k=2;k<a;k+=2){
					if(used[i][i+k-1] && used[i+k][i+a-1]) used[i][i+a-1] = 1;
				}
				if(abs(w[i]-w[i+a-1]) <= 1 && used[i+1][i+a-2]) used[i][i+a-1] = 1;
			}
		}
		//cout << used[1][n] << endl;
		fill(dp,dp+305,-1000000);
		dp[0] = 0;
		for(int i=1;i<=n;i++){
			for(int j=1;j<=i;j++){
				if(dp[j-1] < 0) continue;
				int x = 0;
				if(used[j][i]) x = i-j+1;
				dp[i] = max(dp[i],dp[j-1]+x);
			}
		}
		cout << *max_element(dp+1,dp+n+1) <<endl;
	}
}

C++: #include <iostream>
#include <vector>
using namespace std;

int main(){
  int n;
  while(cin >> n, n){
    vector<int> W(n);
    for(int i = 0; i < n; ++i) cin >> W[i];
    vector< vector<int> > DP(n, vector<int>(n,0));
    for(int k = 1; k < n; ++k){
      for(int i = 0; i < n-k; ++i){
        if(DP[i+1][i+k-1] == k-1 && abs(W[i]-W[i+k]) <= 1) DP[i][i+k] = k+1;
        for(int j = i; j < i+k; ++j){
          DP[i][i+k] = max(DP[i][i+k], DP[i][j]+DP[j+1][i+k]);
        }
      }
    }
    cout << DP[0][n-1] << endl;
  }
  return 0;
}