problem in running Runtime.exec command....

Hi,
I need to run a ghostscript command using java. For that I am using Runtime.exec command. when I compile the class get this error :-
java:22: non-static method exec(java.lang.String) cannot be referenced from a static context
Here is my code:-

import java.lang.*;
public class TestGS
{
   public static void main (String[] args)
  {
    String str;
    String str1;
    char c;
    String file1;
    String file2;
    String mainfile;
    file1="C:\\1.pdf";
    file2="C:\\2.pdf";
    mainfile="C:\\3.pdf";
    c= (char) 34;
    str1 = c + mainfile + c + " " + c + file1 + c + " " + c + file2 + c;
    System.out.println ("str = " + str1);
    str = "C:\\gs\\gs8.11\\bin\\gswin32c.exe -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -  sOutputFile=" + str1;
    System.out.println ("str = " + str);
    Runtime.exec(str);
  }
}

Please help.
Kirti