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;

//#define int long long
#define DBG 1
#define dump(o) if(DBG){cerr<<#o<<" "<<o<<endl;}
#define dumpc(o) if(DBG){cerr<<#o; for(auto &e:(o))cerr<<" "<<e;cerr<<endl;}
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define each(it,c) for(auto it=(c).begin();it!=(c).end();it++)
#define all(c) c.begin(),c.end()
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9 + 7);

#define MAX 301

int dp[MAX][MAX];
int w[MAX];

int dfs(int l, int r) {
	if (dp[l][r] != -1)return dp[l][r];
	if (r - l >= 3) {
		if (abs(w[r - 1] - w[l]) <= 1 && dfs(l + 1, r - 1) == r - l - 2) 
			return dp[l][r] = r - l;
		else {
			for (int m = l + 1; m < r; m++)
				dp[l][r] = max(
					dp[l][r],
					dfs(l, m) + dfs(m, r)
				);
			return dp[l][r];
		}
	}
	if (r - l == 2)
		if (abs(w[r - 1] - w[l]) <= 1)
			return dp[l][r] = 2;
		else
			return dp[l][r] = 0;
	if (r - l <= 1)return dp[l][r] = 0;
}

signed main() {
	for (int n; cin >> n&&n;) {
		memset(dp, -1, sizeof(dp));
		for (int i = 0; i < n; i++)cin >> w[i];
		cout << dfs(0, n) << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <set>
#include <map>
#include <string.h>
#include <cmath>
 
using namespace std;
typedef long long i64;
typedef long double ld;
typedef pair<i64,i64> P;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);++(i))
 
int main()
{
    int n;
    while(cin >> n , n)
    {
        vector<int> w(n,0);
        for(int i = 0;i < n;i++)
        {
            cin >> w[i];
        }
         
        vector<vector<int>> dp(n + 1,vector<int>(n + 1,0));
 
        for(int W = 2;W <= n;W++)
        {
            //left
            for(int l = 0;l < n;l++)
            {
                int r = l + W - 1;
                if(r > n) continue;
 
                if(dp[l + 1][r - 1] == W - 2 && abs(w[l] - w[r - 1]) <= 1)
                    dp[l][r] = W;
 
                for(int mid = l;mid <= r;mid++)
                {
                    dp[l][r] = max(dp[l][r] , dp[l][mid] + dp[mid][r]);
                }
 
            }
 
        }
 
        cout << dp[0][n] << endl;
    }
    return 0;
}

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

// Shrotening
#define fst first
#define snd second
#define pb push_back
#define mp make_pair

// Loop
#define FOR(i,a,b) for(auto i=(a);i<(b);++i)
#define RFOR(i,a,b) for(auto i=(a);i>=(b);--i)

#define REP(i,a) for(long i=0;i<(a);++i)
#define RREP(i,a) for(long i=(a);i>=0;--i)

#define EACH(i,a) for(auto (i)=(a).begin(),_END=(a).end();i!=_END;++i)
#define REACH(i,a) for(auto (i)=(a).rbegin(),_END=(a).rend();i!=_END;++i)

//Algorithm
#define ALL(a) (a).begin(), a.end()
#define RALL(a) (a).rbegin(), a.rend()
#define EXIST(a,x) ((a).find(x)!=(a).end())
#define SORT(a) std::sort((a).begin(), (a).end())
#define UNIQUE(a) std::sort((a).begin(), a.end()), a.erase(std::unique((a).begin(), a.end()), a.end());
#define SUM(a) std::accumulate((a).begin(), (a).end(), 0);

//Setting
#define OPT std::cin.tie(0);std::ios::sync_with_stdio(false);

//debug message
bool debug = true;
#define MSG(s)   if(debug){std::cout << s << std::endl;}
#define DEBUG(x) if(debug){std::cout << "debug(" << #x << "): " << x << std::endl;}

//alias
typedef long long LL;
typedef std::vector<char> VC;
typedef std::vector<int>  VI;
typedef std::vector<long> VL;
typedef std::vector<long long> VLL;

typedef std::vector< VC > VC2;
typedef std::vector< VI > VI2;
typedef std::vector< VL > VL2;
typedef std::vector< VLL > VLL2;

typedef std::pair<int,int> PII;


int n;
VI w(301);
VI2 memo(301, VI(301));

int dp(int l, int r) {
    if(l == r) return 0;
    if(r-l == 1) return (std::abs(w[l]-w[r]) <= 1) ? 2 : 0;
    if(memo[l][r] != -1) return memo[l][r];

    int ans = 0;
    if(std::abs(w[l]-w[r]) <= 1) {
        int res = dp(l+1, r-1);
        if(res == r-l-1) {
            ans = 2 + res;
        }
    }
    for(int i=l; i<r; ++i) {
        ans = std::max(ans, dp(l, i) + dp(i+1, r));
    }
    return memo[l][r] = ans;
}

int main() {
    while(std::cin >> n, n != 0) {
        REP(i, n) {
            std::cin >> w[i];
        }
        memo.assign(301, VI(301, -1));
        std::cout << dp(0, n-1) << std::endl;
    }
}