In the homework for calculating the value of the expression

$$
2-3+4-5+6-7+8-9+10=
$$

Radek forgot to write two parentheses, so he got a result that was 18 greater than what he would have obtained if he had written the parentheses. Add the parentheses in two ways and write down what number Radek got and what number he should have gotten.

(M. Dillingerová)
Radek calculated

$$
2-3+4-5+6-7+8-9+10=6 \text {. }
$$

He got a result 18 more than it should have been. The result should have been $6-18=-12$. To get a smaller number, it is necessary to subtract a larger value, i.e., place the parentheses after the minus sign. Two required solutions:

$$
\begin{aligned}
& 2-3+(4-5)+6-(7+8-9+10)=-12 \\
& 2-3+4-(5+6-7)+8-(9+10)=-12
\end{aligned}
$$

Another correct solution is, for example,

$$
(2-3)+4-5+6-(7+8-9+10)=-12,
$$

etc. Each additional student solution must always be checked to ensure it is correct.