Here
n+module(n-1)
recursive call is module(n-1) only and n+module(n-1) is a constant addition
so recurrence realtion$= T(n)= T(n-1) + c$
$ = T(n-2) +c +c= T(n-2) +2c$
$ = T(n-k) + k \times c $
here $n-k=1 \Rightarrow k=n-1$
$ = T(n-n+1) + (n-1)*c$
$=T(1) + (n-1)*c$
$ =O(n)$