let indexStr (s2 : string) (s1 : string) : int =
let rec inString s1 s2 l1 l2 =(
if (l1 < l2) then -1
else(if(l1 = l2) then (if (s1=s2)
then l1
else -1)
else(if((String.sub s1 0 l2)=s2)
then l1
else inString (String.sub s1 1 (l1-1)) s2 (l1-1) l2))) in
let l1 = String.length s1 in
let in_string = (inString s1 s2 l1 (String.length s2) ) in
if (in_string=(-1))
then (-1)
else (l1-in_string)