OCaml : for loop inside if Statement

I'm newbie in OCaml, what's wrong here (Synthax error):

let summry var3 =
    if var3 > 1 then 
        var4=var3
        for i = var4-1 downto 0 do var3=var3+i done;; 
    else if var3 == 1 then var3 = 1;;
    else var3 = 0 ;;

Is possibile the for loop inside if ?