You will be given a definition of a task first, then some input of the task.
Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: One Saturday morning Ann was going on business. Before starting, she took her son Sam to her friends, Bill and Kate, and asked them to look after him for the day. Sam, an independent boy of six years old, had black hair and was wearing a pair of jeans and a bright yellow shirt. At eleven o'clock Bill took Sam into town. Sam walked along behind Bill, looking at the windows of all the shops they passed. After lunch they went to the park and walked about. The whole place was very crowded. Bill stopped to buy some chocolate. When he turned round to give some to Sam, the boy wasn't there. Bill ran round calling Sam, but he couldn't find him. About an hour later, just in front of a shop, he saw the black hair, the pair of jeans and the bright yellow shirt. The boy looked as if he was waiting for someone to come out of the shop. Bill ran across the road, took the boy by the hand and hurried off, as it was quite late. The boy began to cry and pull hard, but Bill, who didn't know much about children, couldn't understand a word the boy was saying. All the boy made was a lot of noise. When they got home at last, Bill went straight to the kitchen, with the boy still crying. At the door Bill stopped with an amazed look. There, sitting at the kitchen table with Kate, was a small boy with black hair, in a pair of jeans and a bright yellow shirt.
Question: What did Bill buy for Sam?
Output:
chocolate.