  We consider a centered random walk with finite variance and investigate the
asymptotic behaviour of the probability that the area under this walk remains
positive up to a large time $n$. Assuming that the moment of order $2+\delta$
is finite, we show that the exact asymptotics for this probability are
$n^{-1/4}$. To show these asymptotics we develop a discrete potential theory
for the integrated random walk.
