problem link here :- https://www.codechef.com/COOK97A/problems/GUZAC

for test case :- n=3 k=1 x=2 p1=999999999 the resultant answer is 3000000000(1000000000 + 1000000001+999999999) (as per all accepted codes), but should it not be 2999999997 (999999998+1000000000+999999999) as clearly given pi<=1000000000. I tried hard to understand but couldnt ! can anyone explain me this question ?

Recently, I read about treap and found it really amazing. But, from most of the sources im unable to understand its working . can anyone give me good link to understand treap and a list of some good questions so i get good drip over treap. thanks in advance.