请教一个HelloWorld的问题。。。谢谢
大家好，我刚装完了Android的环境，
想测试一个helloworld程序：

package com.yongfeng.helloworld;

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

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

结果提示 R can not be resolved.

如果我引入了包：import android.R;

package com.yongfeng.helloworld;

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

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

可是R.layout.main仍然报错，请问是不是哪里我没有配置好呢？

谢谢