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++: 
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define rep2(i,a,b) for(ll i=a;i<=b;++i)
#define rep(i,n) for(ll i=0;i<n;i++)
#define rep3(i,a,b) for(ll i=a;i>=b;i--)
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pq priority_queue
#define pb push_back
#define eb emplace_back
#define veci vector<int>
#define vecll vector<ll>
#define vecpii vector<pii>
#define vec2(a,b) vector<vec>(a,vec(b))
#define vec2ll(a,b) vector<vec>(a,vecll(b))
#define vec3(a,b,c) vector<vector<vec>>(a,vec2(b,c))
#define vec3ll(a,b,c) vector<vector<vecll>>(a,vec2ll(b,c))
#define fi first
#define se second
#define all(c) begin(c),end(c)
#define ios ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define lb(c,x) distance(c.begin(),lower_bound(all(c),x))
#define ub(c,x) distance(c.begin(),upper_bound(all(c),x))


using namespace std;
int in() {int x;cin>>x;return x;}
ll lin() {ll x;cin>>x;return x;}
template<class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
template<class T> inline void print(pair<T,T> p){cout<<"("<<p.first<<","<<p.second<<") ";}
//template<class T> inline void print(vector<pair<T,T>> v){for(auto e:v)print(e); cout<<endl;}
//template<class T> inline void print(T v){for(auto e:v)cout<<e<<" ";cout<<endl;}
template<typename T>
istream& operator >> (istream& is, vector<T>& vec){
    for(T& x:vec) is >> x;
    return is;
}
const ll INF=1e9+7;

ll dp[305][305];

int main()
{
    while(true){
        ll n;
        cin >> n;
        if(n==0) break;
        vector<ll> w(n);
        rep(i,n) cin >> w[i];
        rep(i,305){
            rep(j,305){
                dp[i][j]=0;
            }
        }
        for(ll W=2;W<=n;W++){
            rep(l,n){
                ll r=l+W;
                if(r>n) continue;
                if(dp[l+1][r-1]==W-2 && abs(w[l]-w[r-1])<=1){
                    dp[l][r]=W;
                }
                for(ll mid=l;mid<=r;mid++){
                    dp[l][r]=max(dp[l][mid]+dp[mid][r],dp[l][r]);
                }
            }
        }
        cout << dp[0][n] << endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define INF 1e9

int main(){

    int n, a, b; cin >> a >> b;
    n = abs(a - b);
    cout << n << endl;
    return 0;
}bool take[310][310];
int dp[310][310];

signed main() {
    int n;
    while(cin >> n, n) {
        int w[310];
        rep(i,0,n) cin >> w[i];
        memset(take, false, sizeof(take));
        memset(dp, 0, sizeof(dp));

        rep(i,0,n) {
            rep(j,0,n-i) {
                int len = i+1;
                int s = j, t = j+i;
                rep(k,s,t) {
                    if(take[s][k] && take[k+1][t]) take[s][t] = true;
                }
                if(len == 2) {
                    if(abs(w[s] - w[t]) < 2) {
                        take[s][t] = true;
                    }
                }
                if(take[s][t]) {
                    int x = s-1, y = t+1;
                    while(1) {
                        if(x < 0 || x >= n || y < 0 || y >= n) break;
                        if(abs(w[x] - w[y]) >= 2) break;
                        take[x][y] = true;
                        x--; y++;
                    }
                }
            }
        }

        rep(i,0,n) rep(j,i+1,n) {
            if(take[i][j]) dp[i][j] = j-i+1;
        }

        rep(i,0,n) rep(j,i,n) rep(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 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;
                    }
                }
                {
                    bool ok = true;
                    for (int l2=l; l2<l+len; l2+=2) {
                        if (!dp[l2][l2+1]) {
                            ok = false;
                            break;
                        }
                    }
                    if (ok) {
                        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;
    }
}