<p>Input the following formula in <code>C1</code>:</p>
<p><code>=IF(ISNA(A1),B1,A1*B1</code>)</p>
<p><em><strong>Screenshots:</strong></em></p>
<p><strong>When <span class="hashtag-raw">#N</span>/A:</strong></p>
<p><strong>When not <span class="hashtag-raw">#N</span>/A:</strong></p>
<p>Let us know if this helps.</p>