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 rep(i,n) for(int i=0;i<(int)n;i++)

int w[300];
int dp[301][301];

int delta(int x,int y){
  if(abs(w[x]-w[y])<=1&&x!=y)return 2;
  return 0;
}

int main(){
  while(1){
    int n;
    cin >> n;
    if(!n)break;
    rep(i,n)cin >> w[i];
    rep(i,301)rep(j,301)dp[i][j]=0;
    rep(i,n)dp[i][i+1]=delta(i,i+1);
    for(int d=2;d<n;d++){
      for(int i=0;i<n-d;i++){
        int tmp = 0,tmp1,tmp2,tmp3;
        for(int k=i;k<i+d;k++){
          if(k>0&&dp[i+1][k-1]==k-i-1)tmp1 = max(dp[i][k],dp[i+1][k-1]+delta(i,k));
          else tmp1 = dp[i][k];
          if(k+2<n&&dp[k+2][i+d-1]==i+d-k-2)tmp2 = max(dp[k+1][i+d],dp[k+2][i+d-1]+delta(k+1,i+d));
          else tmp2 = dp[k+1][i+d];
          if(d%2!=0&&dp[i+1][i+d-1]==d-1)tmp3 = dp[i+1][i+d-1]+delta(i,i+d);
          else tmp3 = 0;
          // cout << i << " " << k << " " << i+d << " " << tmp1 << " " << tmp2 << " " << tmp3 << endl;
          tmp = max(tmp,tmp1+tmp2);
          tmp = max(tmp,tmp3);
        }
        dp[i][i+d] = tmp;
        // cout << tmp << endl;
      }
    }
    cout << dp[0][n-1] << endl;
  }
  return 0;
}

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

using namespace std;
typedef long long ll;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)

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;
				if(j >= n) continue;

				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[0][n-1] << endl;
	}

	return 0;
}

C++: #include <bits/stdc++.h>
  
using namespace std;
  
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define pb push_back
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second
  
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
const int INF = 1e9;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;

bool used[310][310];
bool dp[310][310];
vi w;

int dp2[310][310];
template<typename T> T chmax(T& a, const T& b){return a = max(a, b);}
template<typename T> T chmin(T& a, const T& b){return a = min(a, b);}
//[l, r) de tatakeruka
bool calc(int l, int r) {
	if(r - l <= 1) return false;
	if(used[l][r]) return dp[l][r];
	used[l][r] = true;
	if(r - l == 2) {
		return dp[l][r] = (abs(w[l] - w[r-1]) <= 1);
	}
	for(int m = l + 1; m < r; m++) {
		bool f = true;
		f &= calc(l, m);
		f &= calc(m, r);
		dp[l][r] |= f;
	}
	if(abs(w[l]-w[r-1]) <= 1) dp[l][r] |= calc(l+1, r-1);
	return dp[l][r];
}

int calc2(int l, int r) {
	if(dp2[l][r] != -1) return dp2[l][r];
	if(r - l <= 1) return dp2[l][r] = 0;
	if(dp[l][r]) return dp2[l][r] = r - l;
	int ret = 0;
	for(int m = l + 1; m < r; m++) {
		chmax(ret, calc2(l, m)+calc2(m, r));
	}
	return dp2[l][r] = ret;
}

int main(){
	int n;
	while(cin >> n && n) {
		w.clear();
		w.resize(n);
		rep(i, n) cin >> w[i];
		rep(i, 310) rep(j, 310) dp[i][j] = used[i][j] = false, dp2[i][j] = -1;
		REP(r, 1, n+1) rep(l, r) calc(l, r);
		cout << calc2(0, n) << endl;
		
	}
	return 0;
}

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

#define DUMP(x) cerr << #x << "=" << x << endl
#define DUMP2(x, y) cerr<<"("<<#x<<", "<<#y<<") = ("<<x<<", "<<y<<")"<< endl
#define BINARY(x) static_cast<bitset<16> >(x)

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,m,n) for (int i=m;i<(int)(n);i++)

#define in_range(x, y, w, h) (0<=(int)(x) && (int)(x)<(int)(w) && 0<=(int)(y) && (int)(y)<(int)(h))
#define ALL(a) (a).begin(),(a).end()

typedef long long ll;
const int INF   = 1e9;
const ll  INFLL = 1e18;
typedef pair<int, int> PII;
int dx[4]={0, -1, 1, 0}, dy[4]={-1, 0, 0, 1};

// #define DEBUG

const int MAX_N = 300;
int main()
{
    ios::sync_with_stdio(false);

    int N;
    while (cin >> N, N) {
        vector<int> W(N);
        rep(i, N) cin >> W[i];

        bool dp[MAX_N][MAX_N] = {};
        rep(i, MAX_N) {
            dp[i][i] = true;
        }

        for (int len=2; len<=N; len+=2) {
            for (int l=0; l<=N-len; l++) {
                {
                    int inner_len = len - 2;
                    if (!(inner_len > 0 && !dp[l + 1][l + inner_len]) && abs(W[l] - W[l+ (len - 1)]) <= 1) {
                        dp[l][l + (len - 1)] = true;
                    }
                }
                {
                    int len2 = len - 2;
                    if ((dp[l][l+1] && dp[l+2][l+2+len2-1]) ||
                        (dp[l][l+len2-1] && dp[l+len2][l+len2+1])) {
                        dp[l][l+len-1] = true;
                    }
                }
            }
        }

        int dp2[MAX_N + 1] = {};
        rep(r, N) {
            if (r > 0) dp2[r] = dp2[r-1];
            for (int len=2; len<=r+1; len+=2) {
                int l = r - len + 1;
#ifdef DEBUG
                // DUMP(len);
                // DUMP2(l, r);
#endif
                if (dp[l][r]) {
                    if (l-1 >= 0) {
                        dp2[r] = max(dp2[r], dp2[l-1] + len);
                    } else {
                        dp2[r] = max(dp2[r], len);
                    }
                }
            } 
        }

#ifdef DEBUG
        rep(l, N) {
            rep(r, N) cerr << dp[l][r] << " ";
            cerr << endl;
        }
        cerr << "-----------------------------" << endl;
        rep(i, N) cerr << dp2[i] << " ";
        cerr << endl;
#endif

        cout << dp2[N-1] << endl;
    }
}