GPA calculator in RUBY

#!/usr/bin/env ruby

=begin
Prgram : CGPA calculator
Date : May 16, 2014
Author : ManiG
License : GPL2.0
Version : 1.0
=end

sum=0.0
grade = {"S"=>10,"A"=>9,"B"=>8,"C"=>7,"D"=>6,"E"=>5,"U"=>0}
tot_credits = 0
tot_egrades = 0
print "enter no. of subjects : "
sub_count = gets().to_i
sub_count.times do |c|
    print "enter subject#{c+1}'s credit : "
    credit = gets().to_i
    tot_credits += credit
    print "enter subject#{c+1}'s earned grade :(only 'S','A','B','C','D','E','U') : "
    e_grade = gets().chomp.upcase
    tot_egrades += grade[e_grade] 
    sum += grade[e_grade]*credit
end
gpa = sum/tot_credits
puts
puts "Credits Earned #{tot_credits}"
puts "Grade points earned #{tot_egrades}"
puts "GPA #{format("%.4f",gpa)}"
puts "CGPA #{format("%.4f",gpa)}"
puts
puts "Percentage [=(cgpa*10)-7.5] #{format("%.4f",(gpa*10)-7.5)}"