how to get the length of (b.html>body>div) into a.html

how to get the length of (b.html>body>div) into a.html

can anyone help write a jquery snippet to carry out the function blow?
thank you very much

the code of a.html:
<body> 
    <div id="get-b-length">get (b.html>body>div).length here</div> 
    <button onclick="getlength()">get it now!</button>
</body>


the code of b.html
<body> 
    <div>1111</div> 
    <div>2222</div> 
    <div>3333</div>
</body>


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