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<algorithm>
#include<climits>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<ctime>
#include<string>
#include<cstring>
#include<vector>
#include<stack>
#include<queue>
#include<set>
#include<map>
#include<bitset>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<long long, int> ll_i;

#define PI 3.141592653589793238462643383279
#define mod 1000000007LL
#define rep(i, n) for(i = 0;i < n;++i)
#define rep1(i, n) for(i = 1;i < n;++i)
#define per(i, n) for(i = n - 1;i > -1;--i)
#define int(x) int x; scanf("%d",&x)
#define int2(x, y) int x, y; scanf("%d%d",&x, &y)
#define int3(x, y, z) int x, y, z; scanf("%d%d%d",&x, &y, &z)
#define int4(v, x, y, z) int v, x, y, z; scanf("%d%d%d%d", &v, &x, &y, &z)
#define int5(v, w, x, y, z) int v, w, x, y, z; scanf("%d%d%d%d%d", &v, &w, &x, &y, &z)
#define pri(x) cout << (x) << "\n"
#define pri2(x, y) cout << (x) << " " << (y) << "\n"
#define pri3(x, y, z) cout << (x) << " " << (y) << " " << (z) << "\n"
#define pb push_back
#define mp make_pair
#define all(a) (a).begin(),(a).end()
#define kabe puts("---------------------------")
#define kara puts("")
#define debug(x) cout << " --- " << (x) << "\n"
#define debug2(x, y) cout << " --- " << (x) << " " << (y) << "\n"
#define debug3(x, y, z) cout << " --- " << (x) << " " << (y) << " " << (z) << "\n"
#define X first
#define Y second
#define eps 0.0001
#define prid(x) printf("%.15lf\n", x)

int w[300], dp[301][301];
signed main(void){
  int i, j, k;
  for(int testcase = 0;testcase >= 0;testcase++){
    int(n);
    if(!n)break;

    rep(i, n)scanf("%d", w+i);
    rep(i, n + 1)rep(j, n + 1)dp[i][j] = 0;

    rep(i, n - 1)dp[i][i + 2] = (abs(w[i + 1] - w[i]) <= 1) * 2;

    for(j = 3;j <= n;++j)rep(i, n - j + 1){
      rep(k, j)dp[i][i + j] = max(dp[i][i + j], dp[i][i + k] + dp[i + k][i + j]);
      if(abs(w[i] - w[i + j - 1]) <= 1 && dp[i + 1][i + j - 1] == j - 2)dp[i][i + j] = j;
    }
    //rep(i, n + 1)rep(j, n + 1)if(dp[i][j] > 0)pri3(i, j, dp[i][j]);
    pri(dp[0][n]);

  }
  return 0;
}