Should I lose fat or gain mass/muscle

Hello, new on the forum. I trained several months ago, i stopped training around 4 months and a half. Now I have the question since start to traing again, should I first do a fat lose program/diet or muscle gain. I consider that I have belly but also people around me says to me that I am thin. Which or those is reccommended to start?

My weight is 77 kg or 170 lbs.
And my height is 173 cm or 5.6 feet or 68 inches.

Thanks.

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