title: City cannot be converted to summable
body: <p>So I'm supposed to use a "Summable" interface to add up the populations of the cities. I've been staring at it for an hour but still can't find my error. Please Help! </p>
  
  <p>This is my tester</p>
  
  <pre><code>public class SummableTester extends ConsoleProgram
  {
      public void run()
      {
          City cookie = new City("Coookie", 20000);
          City taco = new City("Taco", 10000);
          System.out.println(taco.getValue());
      }
  }
  </code></pre>
  
  <p>City Class:</p>
  
  <pre><code>    public class City
      {
      private String name;
      private int population;
  
      public City(String name, int population)
      {
          this.name = name;
          this.population = population;
      }
  
      public String getName()
      {
          return this.name;
      }
      public int getValue()
      {
          return this.population;
      }
      public int add(Summable other)
      {
          return getValue() + other.getValue();
      }
  }
  </code></pre>
  
  <p>Summable:</p>
  
  <pre><code>public interface Summable
  {
      public int add(Summable other);
  
      public int getValue();
  }
  </code></pre>