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;

int dp[310][310]; //dp[i][j] は区間[i, j)で叩き出せる数
vector<int> w;

int rec(int l, int m){ //区間DP
    if(dp[l][m] != -1) return dp[l][m];
    
    if(abs(l - m) <= 1) return 0;
    
    int res = 0;
    
    if(abs(w[l] - w[m - 1]) <= 1 && rec(l + 1,m - 1) == m - l - 2) res = m - l;
    
    for(int mid = l + 1; mid < m; mid++){
        res = max(res, rec(l, mid) + rec(mid, m));
    }
    return dp[l][m] = res;
}

int main(){
    while(true){
        int k; cin >> k;
        if(k == 0) break;
        w.resize(k);
        for(int i = 0; i < k; i++){
            cin >> w[i];
        }
        memset(dp, -1, sizeof(dp));
        cout << rec(0, k) << endl;
    }

}

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

using namespace std;

inline int abs(int a, int b)
{
    return a<b ? b-a : a-b;
}

int w[301];
bool rm[301][301];
int rmct[301][301];

int main()
{
    int n;
    while(cin >> n, n)
    {
        for(int i=0; i<n; i++) cin >> w[i];
        for(int i=0; i<=n; i++)
            for(int j=0; j<=n; j++)
                rm[i][j] = i==j;
        for(int r=2; r<=n; r+=2)
        {
            for(int i=0; i+r<=n; i++)
            {
                for(int s=2; s<r; s+=2)
                {
                    if(rm[i][i+s] && rm[i+s][i+r]) rm[i][i+r] = true;
                }
                if(!rm[i][i+r] && rm[i+1][i+r-1] && abs(w[i], w[i+r-1])<=1)
                    rm[i][i+r] = true;
            }
        }
        /*
        int rmw = -1;
        for(int rmr = n / 2 * 2; rmw==-1 && rmr >= 0; rmr-=2)
        {
            for(int i=0; i+rmr<=n; i++)
            {
                if(rm[i][i+rmr])
                {
                    rmw = rmr;
                    break;
                }
            }
        }
        */
        for(int y=0; y<=n; y++)
            for(int x=0; x<=n; x++)
                rmct[y][x] = rm[y][x] ? x - y : 0;
        for(int r=1; r<=n; r++)
        {
            for(int i=0; i+r<=n; i++)
            {
                for(int s=1; s<r; s++)
                {
                    rmct[i][i+r] = max(rmct[i][i+r], rmct[i][i+s]+rmct[i+s][i+r]);
                }
            }
        }
        cout << rmct[0][n] << endl;
        // XXX DEBUG
        /*
        printf("i\\j ");
        for(int x=0; x<=n; x++) printf("%3d ", x);
        cout << endl;
        for(int y=0; y<=n; y++)
        {
            printf("%3d ", y);
            for(int x=0; x<=n; x++)
            {
                printf("%3d ", rm[y][x]);
            }
            cout << endl;
        }
        */
        // XXX DEBUG
        //cout << rmw << endl;
    }
}

C++: #include <iostream>
#include <ccomplex>
#include <vector>
#include <cassert>
#include <utility>
#include <algorithm>
#include <string>
#include <deque>
#include <queue>
#include <functional>
#include <cmath>
#include <iomanip>
#include <map>
#include <numeric>
#include <list>
#include <assert.h>
#include <math.h>
#include <valarray>
#include <stdio.h>
#include <algorithm>
#include <set>
#include <complex>
#include <list>
#include <time.h>
#include <stack>
#include <locale>
#include <clocale>
#include <ctype.h>
#include <wchar.h>

using namespace std;
typedef long long int LL;
typedef pair<long long int, long long int> pii;
typedef pair<double, double> pdd;

#define SORT(c) sort((c).begin(),(c).end())
#define BACKSORT(c) sort((c).begin(),(c).end(),std::greater<LL>())
#define FOR(i,a,b) for(LL i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)


int xcomp(vector<LL>& a, vector<LL>& b) {
	return a[0] < b[0];
}

#define XSORT(c) sort((c).begin(),(c).end(),xcomp)

#define ALL(x) (x).begin(),(x).end()
LL mod = 1000000007;

bool hantei(LL a, LL b) {
	if (a + 1 == b) {
		return true;
	}
	else
		if (a == b) {
			return true;
		}
		else
			if (a == b + 1) {
				return true;
			}
			else {
				return false;
			}
}

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

		vector<LL> num(n, 0);
		vector<LL> v(n, -9999);

		REP(i, n) {
			cin >> v[i];
		}
		REP(i, n - 1) {
			if (hantei(v[i], v[i + 1])) {
				num[i] = 2;
			}
		}

		bool flag = true;

		while (flag) {
			flag = false;

			REP(i, n) {
				LL hoge = num[i];
				if (i + num[i]<n) {
					if (hoge < num[i] + num[i + num[i]]) {
						hoge = num[i] + num[i + num[i]];
					}
				}
				if (i + 1<n) {
					if (i + num[i + 1] + 1<n) {
						if (hantei(v[i], v[i + num[i + 1] + 1])) {
							if (hoge < num[i + 1] + 2) {
								hoge = num[i + 1] + 2;
							}
						}a
					}
				}
				if (hoge != num[i]) {
					flag = true;
					num[i] = hoge;
				}
			}
		}


		vector<LL> fuga(n * 3, 0);

		for (int i = n - 1; i >= 0; --i) {
			fuga[i] = max(fuga[i + 1], num[i] + fuga[i + num[i]]);
		}

		cout << fuga[0] << endl;
	}

}

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

using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
typedef pair<ll, ll> Pll;
typedef vector<int> Vi;
//typedef tuple<int, int, int> T;
#define FOR(i,s,x) for(int i=s;i<(int)(x);i++)
#define REP(i,x) FOR(i,0,x)
#define ALL(c) c.begin(), c.end()
#define DUMP( x ) cerr << #x << " = " << ( x ) << endl

const int dr[4] = {-1, 0, 1, 0};
const int dc[4] = {0, 1, 0, -1};

int dp[310][310];

int main() {
  // use scanf in CodeForces!
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  while (true) {
    int N; cin >> N;
    if (not N) break;
    vector<int> W(N); REP(i, N) cin >> W[i];
    memset(dp, 0, sizeof(dp));
    FOR(w, 2, N + 1) REP(left, N - w + 1) {
      // [left, right]
      int right = left + w - 1;
      if (dp[left + 1][right - 1] == w - 2 and abs(W[left] - W[right]) <= 1) {
        dp[left][right] = w;
      } else {
        FOR(mid, left + 1, right) {
          dp[left][right] = max(dp[left][right], dp[left][mid] + dp[mid + 1][right]);
        }
      }
    }
    cout << dp[0][N - 1] << endl;
  }

  return 0;
}