What is the value of the following expression: $1 - 3 + 5 - 7 + 9 - \cdots - 43 + 45 - 47 + 49$ ?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
What is the value of the following expression: $1 - 3 + 5 - 7 + 9 - \cdots - 43 + 45 - 47 + 49$ ?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
25
What is the value of the following expression: $1 - 3 + 5 - 7 + 9 - \cdots - 43 + 45 - 47 + 49$ ?
train