How do I combine my shortcodes?

Example I want combine shortcode for first paragraph and dropcap.

function st_dropcap( $atts, $content = null ) {
    return '<span class="dropcap">'.$content.'</span>';
}
add_shortcode('dropcap', 'st_dropcap');

function st_paragraph( $atts, $content = null ) {
    return '<p class="first-paragraph">'.$content.'</p>';
}
add_shortcode('paragraph', 'st_paragraph');

On post I tried something like this

[paragraph][dropcap]W[/dropcap]elcome to my blog[/paragraph]

Only paragraph is working. How do I combine this code?

Let me know