<?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";
    }