Concatenating an Android's string resource with a hard-coded string

I am working in an Android project in which I created a string resource in the values/strings.xml file. Let's say this is the string:

<string name="some_text">HELLO WORLD</string>

I can call this string from anywhere inside my project like so:

Toast.makeText(MyActivity.this,
                R.string.some_text,
                Toast.LENGTH_SHORT).show();

This generates a Toast with the text "HELLO WORLD" in it.

But if I try to concatenate this string with some hard-coded text like this:

Toast.makeText(MyActivity.this,
                R.string.some_text + String.valueOf(123),
                Toast.LENGTH_SHORT).show();

I get this nasty result:

"2131368234234 123"

instead of:

"HELLO WORLD 123".

Does someone know what is happening here?

Thanks