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 SolveDaruma(const vector<int> &w) {
    const int n = w.size();
    vector<vector<int>> s(n, vector<int>(n, 0));

    for (int l = 0; l + 1 < n; ++l)
        if (abs(w[l] - w[l + 1]) <= 1)
            s[l][l + 1] = 2;

    for (int i = 3; i <= n; ++i)
        for (int l = 0; l + i <= n; ++l) {
            const int r = l + i - 1;

            if (abs(w[l] - w[r]) <= 1 && s[l + 1][r - 1] == i - 2)
                s[l][r] = i;

            for (int m = l + 1; m <= r; ++m)
                s[l][r] = max(s[l][r], s[l][m - 1] + s[m][r]);
        }

    return s[0][n - 1];
}

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

    int n;

    while (cin >> n, n) {
        vector<int> w(n);
        for (int i = 0; i < n; ++i)
            cin >> w[i];

        cout << SolveDaruma(w) << '\n';
    }

    return 0;
}

C++: // g++ -std=c++11 a.cpp
#include<iostream>
#include<vector>
#include<string>
#include<algorithm>	
#include<map>
#include<set>
#include<utility>
#include<cmath>
#include<random>
#include<cstring>
#include<queue>
#include<stack>
#include<bitset>
#include<cstdio>
#include<sstream>
#include<iomanip>
#include<assert.h>
#include<typeinfo>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)
using namespace std;
//kaewasuretyuui
typedef long long ll;
//#define int ll
typedef int Def;
//typedef ll Def;
typedef pair<Def,Def> pii;
typedef vector<Def> vi;
typedef vector<vi> vvi;
typedef vector<pii> vp;
typedef vector<vp> vvp;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef pair<Def,pii> pip;
typedef vector<pip>vip;
//#define mt make_tuple
//typedef tuple<int,int,int> tp;
//typedef vector<tp> vt;
template<typename A,typename B>bool cmin(A &a,const B &b){return a>b?(a=b,true):false;}
template<typename A,typename B>bool cmax(A &a,const B &b){return a<b?(a=b,true):false;}
//template<class C>constexpr int size(const C &c){return (int)c.size();}
//template<class T,size_t N> constexpr int size(const T (&xs)[N])noexcept{return (int)N;}
const double PI=acos(-1);
const double EPS=1e-7;
Def inf = sizeof(Def) == sizeof(long long) ? 2e18 : 1e9+10;
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};
int n;
vi in;
int dp[310][310];
int f(int l,int r){
	if(dp[l][r]+inf)return dp[l][r];
	if(r-l<2)return dp[l][r]=0;
	int out=-inf;
	loop(i,l,r-1)if(abs(in[i]-in[i+1])<2){
		out=max(out,f(l,i)+2+f(i+2,r));
	}
	int t=f(l+1,r-1);
	if(t==r-l-2){
		if(abs(in[l]-in[r-1])<2)out=max(out,r-l);
	}
	out=max(out,f(l+1,r));
	out=max(out,f(l,r-1));
//	cout<<l<<" "<<r<<" "<<out<<endl;
	return dp[l][r]=out;
}
int main(){
	while(cin>>n,n){
		in=vi(n);
		rep(i,n)cin>>in[i];
		rep(i,310)rep(j,310)dp[i][j]=-inf;
		cout<<f(0,n)<<endl;;
	}
}

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

int main(){
  int n;
  while(cin >> n, n){
    int w[n];
    for(int i = 0; i < n; ++i)
      cin >> w[i];
    
    int dp[n][n];
    for(int i = 0; i < n; ++i){
      fill(dp[i],dp[i]+n,0);
    }
    
    for(int i = 0; i < n-1; ++i){
      for(int j = 0; j < n; ++j){
	if(j+i+1 > n-1) break;
	if(dp[j+1][j+i] == i && abs(w[j]-w[j+i+1]) < 2){
	  dp[j][j+i+1] = dp[j+1][j+i] + 2;
	}else{
	  for(int k = 0; k < i; ++k){
	    dp[j][j+i+1] = max(dp[j][j+i+1],dp[j][j+k]+dp[j+k+1][j+i+1]);
	  }
	}
      }
    }
    /*
    for(int i = 0; i < n; ++i){
      for(int j = 0; j < n; ++j){
      cout << dp[i][j] << " ";
      }
      cout << endl;
    }
    */
    cout << dp[0][n-1] << endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include<stdio.h>
#include<cstring>
#include<math.h>
#include<map>
#include<bitset>
#include<iomanip>
#include<queue>
#include<functional>
#include<stack>
#include<tuple>
#include<cassert>
using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
using P = pair<int, int>;

int main() {
  while(true) {
    int n;
    int w[300];
    int dp[301][301] = {0};
    cin >> n;
    if(n == 0) break;
    rep(i, n) {
      cin >> w[i];
    }
    rep(i, n-1) {
      if(abs(w[i]-w[i+1]) <= 1) dp[i][i+1] = 2;
      else dp[i][i+1] = 0;
    }

    for(int l = 3; l <= n; l++) {
      for(int i = 0; i + l - 1 < n; i++) {
        int j = i + l - 1;
        if(dp[i+1][j-1] == j - i - 1) {
          if(abs(w[i]-w[j]) <= 1) dp[i][j] = j - i + 1;
          else dp[i][j] = j - i - 1;
        }
        for(int k = i; k <= j-1; k++) {
          dp[i][j] = max(dp[i][j], dp[i][k] + dp[k+1][j]);
        }
      }
    }

    cout << dp[0][n-1] << endl;

  }
  return 0;
}