non static variable cannot be referenced from static context

public class Add{ int foo=7; int bar=8; public int add(int x,int y){ int z=x+y; System.out.println(z); return z; }//add public static void main(String []s){ Add a1=new Add(); a1.add(7,8); Add a2=new Add(); a2.add(foo,bar); }//main }//class

I wrote the following program. When I compiled this I got the error message saying "non static variable cannot be referenced from static context". How to solve this problem?