[CSES] Array Description

void solve()
{
    ll n, m;
    cin >> n >> m;

    vll a(n);
    rep(i, n) cin >> a[i];

    vvll dp(n + 1, vll(m + 1, 0));
    if (a[0])
        dp[0][a[0]] = 1;
    else
        FOR(j, 1, m)
    dp[0][j] = 1;

    FOR(i, 1, n - 1)
    {
        if (a[i])
        {
            FOR(k, -1, 1)
            {
                ll x = a[i] - k;
                if (x > 0 && x <= m)
                    dp[i][a[i]] += dp[i - 1][x];
                dp[i][a[i]] %= mod;
            }
        }
        else
        {
            FOR(j, 1, m)
            {
                FOR(k, -1, 1)
                {
                    ll x = j - k;
                    if (x > 0 && x <= m)
                        dp[i][j] += dp[i - 1][x];
                    dp[i][j] %= mod;
                }
            }
        }
    }

    debug(dp);

    ll ans = 0LL;
    FOR(j, 1, m)
    ans += dp[n - 1][j], ans %= mod;

    cout << ans << "\n";
}