import std.stdio, std.string, std.conv, std.range;
import std.algorithm, std.array, std.typecons, std.container;
import std.math, std.numeric, std.random, core.bitop;

enum inf = 1_001_001_001;
enum infl = 1_001_001_001_001_001_001L;


void main() {
    while (true) {
        int N;
        scan(N);

        if (N == 0) return;

        auto w = readln.split.to!(int[]);
        auto ans = solve(N, w);
        writeln(ans);
    }
}

int solve(int N, int[] w) {
    auto dp = new int[][](N + 1, N + 1);
    fillAll(dp, -1);

    int rec(int l, int r) {
        if (r - l <= 1) {
            return 0;
        }
        if (dp[l][r] != -1) {
            return dp[l][r];
        }
        dp[l][r] = 0;
        foreach (i ; l + 1 .. r) {
            chmax(dp[l][r], rec(l, i) + rec(i, r));
        }
        if (rec(l + 1, r - 1) == r - 1 - (l + 1) && abs(w[l] - w[r - 1]) <= 1) {
            dp[l][r] = r - l;
        }
        return dp[l][r];
    }

    auto ans = rec(0, N);
    return ans;
}












void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}

void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}

bool chmin(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x > arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}

bool chmax(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x < arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}

