<|im_start|>system
You are a helpful assistant. You first think about the reasoning process in the mind and then provide the user with the answer. You should try to truely solve the question by yourself without cheating.  <|im_end|>
<|im_start|>user
 Xiao Ming started adding natural numbers in sequence from 1 on a calculator. When he added up to a certain number, the result was 1991. Later, he found that he had missed adding one number. What is the missed number? Show your work in <think> </think> tags. And return the final answer in <answer> </answer> tags.<|im_end|>
<|im_start|>assistant
Let me solve this step by step.
<think>
user
25
Xiao Ming started adding natural numbers in sequence from 1 on a calculator. When he added up to a certain number, the result was 1991. Later, he found that he had missed adding one number. What is the missed number?
test