How can I combine this two htaccess rules?

I have two conditions for the pages /portfolio and /portfolio/

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RewriteRule ^portfolio$ index.php?p=portfolio&%{QUERY_STRING}
RewriteRule ^portfolio/$ index.php?p=portfolio&%{QUERY_STRING}

Is there a possibility to combine this conditions?

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RewriteRule ^portfolio/?$ index.php?p=portfolio&%{QUERY_STRING}

the ? says "exactly 0 or 1 '/'"