source_code: /*
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   * To change this template file, choose Tools | Templates
   * and open the template in the editor.
   */
  
  
  import java.util.ArrayList;
  import java.util.Scanner;
  
  /**
   *
   * @author Ahmed
   */
  
  
  
  
  public class Watermelon {
  
     static class Passengers {
    
       public int floor ;
       public int time;
       
       
       public Passengers( int floor  , int time){
           this.floor =floor;
           this.time =time;
       }
       
  }
  
      
      /**
       * @param args the command line arguments
       */
      public static void main(String[] args) {
          // TODO code application logic here
          
          Scanner in = new Scanner(System.in);
          
          int x = in.nextInt() , y = in.nextInt();
          
          ArrayList<Passengers> list = new ArrayList<>();
          
          for(int i = 1 ; i <= x ; ++i){
              list.add(new Passengers(in.nextInt(), in.nextInt()));
          }
          
          int sum = 0 ;
         for(int i = list.size() - 1 ; i >= 0 ; --i)
         {
            int s = y -  list.get(i).floor;
            sum = sum + s ; 
            
            if(sum < list.get(i).time)
            {
                sum = sum + ( list.get(i).time - sum);
            }
            
            y = list.get(i).floor;
         } 
          
          
         if( list.get(list.size() - 1).floor != 0){
             sum = sum  + (list.get(0).floor);
         }
          System.out.println(sum);
      }
      
  }
  