ruby regular expressions

I am trying to create a program where a user enters four numbers using regular expressions . If one of those numbers is 13 then the numbers to the left do not count toward the sum. My problem is creating an exception where none of the numbers equal 13. I cant seem to find a regular expression for my exception
thanks in advanced

puts "enter a number then hit enter four times"
 
 number1 = STDIN.gets
 
 number2 = STDIN.gets
 number3 = STDIN.gets
 
 number4 = STDIN.gets
 
 
 if number1 =~ /13/ then
   puts number2.to_i + number3.to_i + number4.to_i
 end
 
 if number2 =~/13/ then
   puts number3.to_i + number4.to_i
 end
 
 if number3 =~/13/ then
   puts number4.to_i
  if number4 =~/13/ then
    puts "0"
  end
  if number1 != 13 or number2 != 13 or number3 != 13 or number4 != 13
    puts number1.to_i + number2.to_i + number3.to_i + number4.to_i
  end
 end
 

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