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 rep(i,n) for(int i = 0;i<n;i++)
using namespace std;
typedef pair<int,int> P;
typedef long long ll;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0;}
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0;}
int n;
vector<int>a;
vector<vector<int> > dp;
int rec(int l = 0,int r = n) {
    if((r-l)<=1) return 0;
    if((r-l)==2) {
        if(abs(a[l]-a[l+1])<=1) {
            return 2;
        } else {
            return 0;
        }    
    }
    if(dp[l][r] != -1)return dp[l][r];
    if(abs(a[l] - a[r-1]) <= 1 && rec(l+1,r-1) == r-l-2) {
        chmax(dp[l][r],r-l);
    }
    for(int i = l+1;i<=r-1;i++) {
        chmax(dp[l][r],rec(l,i) + rec(i,r));
    }
    return dp[l][r];
}
int main() {
    while(true) {
        cin >> n;
        if(n==0) break;
        a.resize(n);
        rep(i,n) cin >> a[i];
        dp.assign(n+2,vector<int>(n+2,-1));
        cout << rec() << endl;
    }
}

C++: #include "bits/stdc++.h"
#define REP(i,n,N) for(int i=(n); i<(N); i++)
#define RREP(i,n,N) for(ll i=(N-1); i>=n; i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define ALL(v) (v).begin(),(v).end()
#define p(s) cout<<(s)<<endl
#define p2(a,b) cout<<(a)<<" "<<(b)<<endl
#define v2(T) vector<vector<T>>
typedef long long ll;
using namespace std;
const ll mod=1e9+7;
const int inf=1e9;

int dp[310][310];
int n;
int w[310];
int main() {
    while(cin>>n, n){
        REP(i,0,n) REP(j,0,n) dp[i][j] = 0;
        REP(i,0,n) cin>>w[i];
        for(int k=1;k<n;k++){
            REP(L,0,n-k){
                int R = L + k;
                if(dp[L+1][R-1] == k-1) dp[L][R] = max(dp[L][R], dp[L+1][R-1] + (abs(w[L]-w[R]) <= 1)*2);
                REP(i,L+1,R-1) dp[L][R] = max(dp[L][R], dp[L][i] + dp[i+1][R]);
            }
        }
        p(dp[0][n-1]);
    }


    return 0;
}

C++: #include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(a,b) for(auto (a): (b))
#define all(v) (v).begin(),(v).end()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define spair(p) cout<<#p<<": "<<p.fi<<" "<<p.se<<endl
#define svec(v) cout<<#v<<":";rep(kbrni,v.size())cout<<" "<<v[kbrni];cout<<endl
#define sset(s) cout<<#s<<":";each(kbrni,s)cout<<" "<<kbrni;cout<<endl
#define smap(m) cout<<#m<<":";each(kbrni,m)cout<<" {"<<kbrni.first<<":"<<kbrni.second<<"}";cout<<endl

using namespace std;

typedef pair<int,int>P;

const int MAX_N = 305;

int dp[MAX_N][MAX_N];

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    while(1){
        int n;
        cin >> n;
        if(n == 0){
            break;
        }
        vector<int> vec(n);
        rep(i,n){
            cin >> vec[i];
        }
        rep(i,n){
            rep(j,n+1){
                dp[i][j] = 0;
            }
        }
        for(int i=2;i<=n;i++){
            rep(j,n-i+1){
                if(dp[j+1][j+i-1] == i-2 && abs(vec[j]-vec[j+i-1]) <= 1){
                    dp[j][j+i] = i;
                }
                for(int k=j+1;k<j+i;k++){
                    dp[j][j+i] = max(dp[j][j+i],dp[j][k]+dp[k][j+i]);
                }
            }
        }
        cout << dp[0][n] << endl;
    }
    return 0;
}

C++: #include <stdio.h>
#include <cmath>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
typedef long long int ll;
#define BIG_NUM 2000000000
#define MOD 1000000007
using namespace std;

int main(){

	int N,data[301];
	bool** table = new bool*[301]; //table[left][right] = left???????§?????????????2*right??????????????¨?¶??????§??????????????????
	int** maximum = new int*[301]; //?????????????????????????????§?¶????????????°
	for(int i = 0; i <= 300; i++){
		table[i] = new bool[151];
		maximum[i] = new int[301];
	}

	bool FLG;

	while(true){
		scanf("%d",&N);
		if(N == 0)break;

		if(N <= 1){
			printf("0\n");
			continue;
		}

		//?¶??????????????????????????????????
		for(int i = 1; i <= N; i++){
			for(int k = 0; k <= N/2; k++)table[i][k] = false;
			for(int k = 1; k <= N; k++)maximum[i][k] = 0;
		}

		for(int i = 1; i <= N; i++)scanf("%d",&data[i]);

		//????????????2???????¨?
		for(int i = 1; i <= N-1; i++){
			if(abs(data[i+1]-data[i]) <= 1){
				table[i][1] = true;
				maximum[i][i+1] = 2;
			}
		}

		//?¶??????????????????????????¨????
		for(int length = 4; length <= N; length += 2){ //??????????????????
			for(int left = 1; left+length -1 <= N; left++){ //??????????????????
				FLG = false;

				if(table[left+1][(length-2)/2] == true && abs(data[left] - data[left+length-1]) <= 1)FLG = true;//??????????????£?????????length-2??????????¶????????????????????????¨??????+length-1?????´???????????????????¶?????????´???

				if(!FLG){
					for(int i = 2; i <= length-2; i += 2){
						if(table[left][i/2] == true && table[left+i][(length-i)/2] == true){
							FLG = true;
							break;
						}
					}
				}
				if(FLG){
					table[left][length/2] = true;
					maximum[left][left+length-1] = length; //????¨?????????§??????OK????????§?????¶????????§???????¨????
				}else{
					//maximum[left][left+length-1]?????´??°????????°????????????
					for(int i = 2; i <= length-2; i+= 2){
						maximum[left][left+length-1] = max(maximum[left][left+length-1],maximum[left][left+i-1]+maximum[left+i][left+length-1]);
					}
				}
			}
		}

		if(N%2 == 0)printf("%d\n",maximum[1][N]);
		else{
			printf("%d\n",max(maximum[1][N-1],maximum[2][N]));
		}
	}

	return 0;
}