However, for a truly random function, the value of the function at the point M is independent of its value at the points M-1 to M-Q. So the best the adversary can do at predicting the value of the function at the point M is just guess the value, because he has no information about F of M. And as a result his advantage if he guesses the value of the function at the point M he'll guess it right with probability exactly one over Y. And then the tag that he produced will be correct with probability exactly one over Y. Okay, again he had no information about the value of the function of M and so the best he could do is guess. 