<?php

while ($line = fgets(STDIN)) {
    $avg = trim($line);
	if ($avg >= 90) 
		$grade = "A";
	elseif ($avg >= 80) 
		$grade = "B";
	elseif ($avg >= 70) 
		$grade = "C";
	else 
		$grade = "F";

	echo "$avg -> $grade\n";
	}