$$23 - [24 - \left\{25 - (26 - \overline{27 - 28})\right\}] = ?$$
$$23 - [24 - \left\{25 - (26 - \overline{27 - 28})\right\}] = ?$$
$$=23 - [24 - \left\{25 - (26 - 1\right\}] $$
$$=23 - [24 - \left\{25 - (25\right\}] $$
$$=23 - [24 ] $$
$$=-1 $$
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