Weird Function

Chef and his friends are locked inside a room, to get out of the room they need to enter a password in a computer panel. This secret password can be obtained by calculating the value of a function $g(n,m)$, value of $n$ and $m$ will be given to you. Help Chef and his friends escape the room. You are given a function $f(n,m)$ which is defined as : ${f(n,m) = \begin{cases} \sum\lim