Unable to install sexp gem

I have ruby 1.8.6 installed on windows.
I have downloaded sexp gem.
I am trying to install the gem,but getting following error-

ERROR: While executing gem … (RuntimeError)
Error instaling sexp-0.1.gem:
sexp requires multi = 0.1

Whats the error? Please help

sexp requires multi = 0.1

That means before you can install the sexp gem, you must install the
multi gem (version 0.1). If you have a recent rubygems installation,
simply typing “gem install sexp” automatically downloads and installs
the multi and sexp gems.