7/18
Since we're talking about the number 18, I assume you're referring to the 18 letters of the English alphabet after "r." In that case, there are 7 letters in the alphabet after "r" that are not "s": "t", "u", "v", "w", "x", "y", and "z".
So the probability of choosing a letter other than "s" from these 18 letters is 7/18. Therefore, the answer is:
7/18