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;
using ll = long long;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define all(x) (x).begin(),(x).end()
#define pb push_back
#define fi first
#define se second
#define dbg(x) cout<<#x" = "<<((x))<<endl
template<class T,class U> ostream& operator<<(ostream& o, const pair<T,U> &p){o<<"("<<p.fi<<","<<p.se<<")";return o;}
template<class T> ostream& operator<<(ostream& o, const vector<T> &v){o<<"[";for(T t:v){o<<t<<",";}o<<"]";return o;}

int main(){
    int n;
    while(cin >>n,n){
        vector<int> w(n);
        rep(i,n) cin >>w[i];

        vector<vector<int>> ok(n,vector<int>(n));
        rep(i,n-1) ok[i][i+1] = (abs(w[i]-w[i+1]) <= 1);

        for(int b=4; b<=n; b+=2){
            for(int l=0; l+b-1<n; ++l){
                int r = l+b-1;
                for(int i=l+1; i<r; ++i) ok[l][r] |= (ok[l][i] & ok[i+1][r]);
                ok[l][r] |= ((abs(w[l]-w[r])<=1) & ok[l+1][r-1]);
            }
        }

        vector<int> dp(n+1,-n);
        dp[0] = 0;
        rep(i,n){
            for(int j=i; j<n; ++j){
                if(ok[i][j]) dp[j+1] = max(dp[j+1], dp[i]+(j-i+1));
            }
            dp[i+1] = max(dp[i+1],dp[i]);
        }
        cout << dp[n] << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
#define MOD 1000000007LL
using namespace std;

typedef long long ll;
typedef pair<int,int> P;

int n;
int w[301];
int dp[301][301];

int func(int l,int r){
	if(dp[l][r]!=-1)return dp[l][r];
	if(l+1>=r)return 0;
	if(l+2==r){
		if(abs(w[l]-w[l+1])<=1)return (dp[l][r]=2);
		else return (dp[l][r]=0);
	}
	int res=0;
	res=max(res,func(l+1,r));
	res=max(res,func(l+2,r)+((abs(w[l]-w[l+1])<=1)?2:0));
	res=max(res,func(l,r-1));
	res=max(res,func(l,r-2)+((abs(w[r-1]-w[r-2])<=1)?2:0));
	if(abs(w[l]-w[r-1])<=1){
		int v=func(l+1,r-1);
		if(v==((r-1)-(l+1))){
			res=r-l;
		}
	}
	return (dp[l][r]=res);
}

int main(void){
	while(1){
		scanf("%d",&n);
		if(n==0)break;
		for(int i=0;i<n;i++){
			scanf("%d",&w[i]);
		}
		memset(dp,-1,sizeof(dp));
		printf("%d\n",func(0,n));
	}
	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() );
		v.erase(unique(v.begin(), v.end()), 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++: //#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define IN(a, b, x) (a<=x&&x<b)
#define MP make_pair
#define PB push_back
#define MOD 1000000007
#define INF (1LL<<30)
#define LLINF (1LL<<60)
#define PI 3.14159265359
#define EPS 1e-12
//#define int ll

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

int w[305], dp[305][305];
signed main(void)
{
	while(true) {
		int n;
		cin >> n;
		if(!n) break;
		REP(i, n) cin >> w[i];
		REP(i, 305) REP(j, 305) dp[i][j] = 0;

		FOR(W, 2, n+1) {
			REP(i, n) {
				int j = i+W-1;
				//cout << i << " " << j << endl;

				if(dp[i+1][j-1] == W-2 && abs(w[i] - w[j]) <= 1) dp[i][j] = W;
				FOR(k, i, j) {
					dp[i][j] = max(dp[i][j], dp[i][k] + dp[k+1][j]);
					//cout << dp[i][j] << endl;
				}
			}
		}
		/*REP(i, n) {
			cout << i << ":";
			REP(j, n) {
				cout << dp[i][j] << " ";
			}
			cout << endl;
		}*/
		cout << dp[0][n-1] << endl;
	}


	return 0;
}