Turn a test average into a letter grade with IFS

A student's four test scores (out of 100) are in B2:B5. In D2, write a formula that works out the AVERAGE of those four scores and returns a letter grade: A for 90 or more, B for 80 or more, C for 70 or more, D for 60 or more, and F for anything below 60. Use IFS.