我正在尝试使用Android tools > Export Signed apllication project生成apk，但我收到此错误：
Proguard return with error code 1. See console
控制台说：
[2013-08-21 17:03:56 - DisplayMetrics] Warning: there were 23 unresolved references to classes or interfaces.
[2013-08-21 17:03:56 - DisplayMetrics] You may need to specify additional library jars (using '-libraryjars').
[2013-08-21 17:03:56 - DisplayMetrics] java.io.IOException: Please correct the above warnings first.
[2013-08-21 17:03:56 - DisplayMetrics] at proguard.Initializer.execute(Initializer.java:321)
[2013-08-21 17:03:56 - DisplayMetrics] at proguard.ProGuard.initialize(ProGuard.java:211)
[2013-08-21 17:03:56 - DisplayMetrics] at proguard.ProGuard.execute(ProGuard.java:86)
[2013-08-21 17:03:56 - DisplayMetrics] at proguard.ProGuard.main(ProGuard.java:492)
这是在我在libs文件夹中添加3个jar文件时发生的。我该如何解决这个问题？