 n, m = gets.split.map(&:to_i)           
 ab = Hash.new(0)                        
 n.times do                              
   a, b = gets.split.map(&:to_i)         
   ab[a] += b                            
 end                                     
 price = ab.sort {|a, b| a[0] <=> b[0] } 
                                         
 ans = 0                                 
 while m > 0 do                          
   c = [m, price[0][1]].min              
   ans += price[0][0] * c                
   price.delete_at(0)                    
   m -= c                                
 end                                     
                                         
 puts ans                                
