Using General Custer to find "half way" date - Errors

I need to find the in between (half way) date between “Start Date” and “Est End Date”. Here is the formula I am using, but I get an error.

DATEADD({{client’s Start Date}}, ROUND(DATEDIF({{client’s Start Date}}, {{client’s Est End Date}}, “days”) / 2), “days”)

The error is: General Caster Formula Error: Wrong number of arguments for ROUND() function: 1 given, 2 expected

TIA for the help!