How Long the Drunk Walk Lasts

A student stands at metre 27 of a 100-metre bridge. Every second he steps one metre forward or one metre backward, each with probability 1/2, independently of every other step. He stops the moment he first reaches metre 0 or metre 100.

He already knows the probability of reaching metre 100 first is 27/100. On average, how many seconds does the walk last?

Answer format: enter an integer.