I am trying to work a RSA problem with my two primes of p=17 and q=23 my n = 31

I computed z = p q = 391

(q-1) (p-1) = 352

I need to compute s in n s mod 352 = 1

I have gone through the examples time and time again and can not figure it out. I think I use the Euclidean Algorithm but I am missing something for sure. Any help would be much appreciated.