so i bin trying to gain weight

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and was asking for a bit of advice or help? im on the skinny side but iv got a bit of muscle, im trying to gain some more weight along with muscle and was wondering if people had any thoughts on the kinda diet i should go on, i was also wondering if anybody has ever tried the weight gainer formula's and milkshake powders that are sold in holland&barrets and if they would recommend me to use them if they are actually helpful or useful to gain weight? thanks.