R 无法解决 - Android 错误

I just downloaded and installed the new Android SDK. I wanted to create a simple application to test drive it.

向导创建了这段代码:

package eu.mauriziopz.gps;

import android.app.Activity;
import android.os.Bundle;

public class ggps extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

但是 Eclipse 给了我错误

R 无法解析

上线

setContentView(R.layout.main);

为什么?

PS:我在 res/layout/ 下确实有一个名为 main.xml 的 XML 文件.

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="your.correct.package.name"
   ...