Is my cell free from malware

Hi

While I was browsing the web a strange site opened with a popup saying that i should download a certain piece of software from the site to prevent my phone from slowing. I could tell it was probably a scam so i immediately shut the site, I then downloaded free Avast Mobile Security and ran it, it stated that my phone was fine. I am worried that there might be malware on my phone regardless because the site was opening before i had Avast. How can I make sure that my phone is safe from malware and adware etc.

Many Thanks

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