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<iostream>
#include<cstdio>
#include<vector>
#include<string>
#include<cstring>
#include<functional>
#include<stacq>
#include<queue>
#include <iomanip>
#include<map>
#include<limits>
#include<cmath>
#include<algorithm>
#include<bitset>
#include<deque>
#include<utility>
#include<complex>
#include<cstdlib>
#include<set>
#include<cctype>

using namespace std;

#define DBG cerr << '!' << endl;
#define REP(i,n) for(ll (i) = (0);(i) < (n);++i)
#define rep(i,s,g) for(ll (i) = (s);(i) < (g);++i)
#define rrep(i,s,g) for(ll (i) = (s);i >= (g);--(i))
#define PB push_bacq
#define MP maqe_pair
#define FI first
#define SE second
#define SHOW1d(v,n) {for(int W = 0;W < (n);W++)cerr << v[W] << ' ';cerr << endl << endl;}
#define SHOW2d(v,i,j) {for(int aaa = 0;aaa < i;aaa++){for(int bbb = 0;bbb < j;bbb++)cerr << v[aaa][bbb] << ' ';cerr << endl;}cerr << endl;}
#define ALL(v) v.begin(),v.end()
#define Decimal fixed<<setprecision(10)

typedef long long ll;
typedef vector<int> iv;
typedef vector<iv> iiv;
typedef vector<string> sv;

int dp[310][310];

int main()
{
	int n;
	
	while(cin >> n,n)
	{
		REP(i,310)REP(j,310)dp[i][j] = 0;
		
		vector<int> v(n);
		REP(i,n)cin >> v[i];
		
		REP(i,n-1)if(abs(v[i]-v[i+1]) < 2)dp[i][i+1] = 2;
		
		for(int q = 2;q < n;q++)
		{
			for(int i = 0;i+q < n;i++)
			{
				int j = i+q;
				for(int k = 0;k < j-i;k++)
				{
					dp[i][j] = max(dp[i][j],dp[i][i+k]+dp[i+k+1][j]);
				}
				if(dp[i+1][j-1] == j-i-1)
				{
					int tmp = 0;
					if(abs(v[i]-v[j])<2)tmp = 2;
					dp[i][j] = max(dp[i][j],j-i-1+tmp);
				}
			}
		}
		
		cout << dp[0][n-1] << endl;
		
	}
	
	
	return 0;
}

C++: #include<iostream>
#include<vector>
#include<tuple>
#include<algorithm>
#include<cmath>
using namespace std;
vector<tuple<int, int, int>>vec; int n, a[1000], dp[1000][1000];
int main() {
	while (true) {
		cin >> n; if (n == 0)break; vec.clear(); for (int i = 0; i < 1000; i++) { for (int j = 0; j < 1000; j++)dp[i][j] = 0; }
		for (int i = 0; i < n; i++)cin >> a[i];
		for (int i = 0; i < n; i++) { for (int j = i + 1; j < n; j++) { vec.push_back(make_tuple(j - i, i, j)); } }
		sort(vec.begin(), vec.end());
		for (int i = 0; i < n - 1; i++) { if (abs(a[i] - a[i + 1]) <= 1) { dp[i][i + 1] = 1; } }
		for (int i = 0; i < vec.size(); i++) {
			int a1 = get<1>(vec[i]), a2 = get<2>(vec[i]);
			if (dp[a1][a2] == 0)continue;
			if (a2 < n - 2 && abs(a[a2 + 1] - a[a2 + 2]) <= 1)dp[a1][a2 + 2] = 1;
			if (a1 > 0 && a2 < n - 1 && abs(a[a1 - 1] - a[a2 + 1]) <= 1)dp[a1 - 1][a2 + 1] = 1;
		}
		for (int i = 0; i < n; i++) { for (int j = 0; j < n; j++) { if (dp[i][j] == 1)dp[i][j] = j - i + 1; } }
		for (int i = n - 1; i >= 0; i--) {
			for (int j = n - 1; j >= i + 1; j--) {
				for (int k = n - 1; k >= j; k--) {
					dp[i][k] = max(dp[i][k], dp[i][j - 1] + dp[j][k]);
				}
			}
		}
		cout << dp[0][n - 1] << endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#define REP(i,n) for (long long i=0;i<(n);i++)
#define FOR(i,a,b) for (long long i=(a);i<(b);i++)
#define RREP(i,n) for(long long i=n;i>=0;i--)
#define RFOR(i,a,b) for(long long i=(a);i>(b);i--)
#define dump1d_arr(array) REP(i,array.size()) cerr << #array << "[" << (i) << "] ==> " << (array[i]) << endl
#define dump2d_arr(array) REP(i,array.size()) REP(j,array[i].size()) cerr << #array << "[" << (i) << "]" << "[" << (j) << "] ==> " << (array[i][j]) << endl
#define dump(x)  cerr << #x << " => " << (x) << endl
#define dumpP(p) cerr << "( " << p.first << " , " << p.second << " )" << ends
#define SORT(c) sort((c).begin(),(c).end())
#define MIN(vec) *min_element(vec.begin(), vec.end())
#define MAX(vec) *max_element(vec.begin(), vec.end())
#define UNIQ(vec) vec.erase(unique(vec.begin(), vec.end()),vec.end()) //ソートの必要あり
#define IN(n,m)  (!(m.find(n) == m.end()))
#define ENUM(m) for (auto itr = m.begin(); itr != m.end(); ++itr)
#define dump_MAP(m) for(auto itr = m.begin(); itr != m.end(); ++itr) { cerr << itr->first << " --> "  << itr->second << endl; }
#define FINDL(vec,x) (lower_bound(vec.begin(),vec.end(),x) - vec.begin())
#define FINDU(vec,x) (upper_bound(vec.begin(),vec.end(),x) - vec.begin())
#define ROUND(N) setprecision(N)
#define ROUND_PRINT(N,val) cout << fixed;cout << setprecision(N) << val << endl
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define INARR(h,w,x,y) (0 <= y && y < h && 0 <= x && x < w)
#define EQ(a,b) (abs(a - b) < 1e-10)
using namespace std;
constexpr int dx[4] = {0,1,0,-1};
constexpr int dy[4] = {1,0,-1,0};
constexpr long double pi = M_PI;
constexpr double eps = 1e-10;
constexpr long mod = 1000000007;
constexpr short shINF = 32767;
constexpr long loINF = 2147483647;
constexpr long long llINF = 9223372036854775807;
typedef long long LL;
typedef vector<LL> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<LL,LL> pr;
typedef vector<bool> VB;
typedef vector<pr> VP;
struct Order {
	bool operator() (pr const& a,pr const& b) const {
		return a.first > b.first || ((a.first == b.first) && (a.second > b.second));
	}
};
int n;
VI w;
typedef bitset<300> bit;
struct Comp {
	bool operator() (const bit &a,const bit &b) {
		return a.to_ullong() < b.to_ullong();
	}
};
map<bit,int,Comp> memo;
int dfs(bit state) {
	if (IN(state,memo)) return memo[state];
	int num = 0,fi;
	REP(i,n-1) {
		if ((!state[i])) {
			fi = i;
			break;
		}
	}
	FOR(i,fi+1,n) {
		if (!state[i]){
			if (abs(w[i] - w[fi]) <= 1) {
				bit next = (state | bit(1<<i) | bit(1<<(fi)));
				num = max(dfs(next)+2,num);
			}
			fi = i;
		}
	}
	return memo[state] = num;
}

int main(void) {
	while (1) {
		cin >> n;
		if (n == 0) break;
		w.resize(n);
		REP(i,n) cin >> w[i];
		cout << dfs(bit(0)) << endl;
		memo.clear();
	}
}