powershell function that prints letter grade with a range

Question

so far i got this need help writing this function:

   if (grades$Final[i] >=90) {
         "A"
      }
      else if (grades$Final[i] >=80){
         "B"
      }
      else if(grades$Final[i] >=70){
         "C"
      }
      else if(grades$Final[i] >=60){
         "D"
      }
      else {"F"}
      }
}