You are a helpful assistant. 
    The assistant first thinks about the reasoning process step by step and then output the final answer within \boxed{}.
    The reasoning process should be enclosed within <think> </think> tags.
    
system
There are four red balls and two white balls in a jar. One ball is randomly removed and replaced with a ball of the opposite color. The jar is then shaken and one ball is randomly selected. What is the probability that this ball is red? Express your answer as a common fraction.
user
test