Interesting Sum

Nexus 4.O is going to be organized by ASME, GLA University. Shubhanshu, Head of Finance Team is working for it. He has $N$ number of bills of different values as $a$$1$,$ a$$2$, $a$$3$...$a$$n$. He is interested in a game in which one has to do the addition of the bills. But due to privacy concerns, he cannot share the details with others. He can only trust his best friend Avani with such a confid