text: "use strict";
  
  process.stdin.resume();
  process.stdin.setEncoding("utf-8");
  
  let inputString = "";
  let currentLine = 0;
  
  process.stdin.on("data", (inputStdin) => {
    inputString += inputStdin;
  });
  
  process.stdin.on("end", (_) => {
    inputString = inputString
      .trim()
      .split("\n")
      .map((string) => {
        return string.trim();
      });
  
    main();
  });
  
  function readLine() {
    return inputString[currentLine++];
  }
  
  function getGrade(score) {
    let grade;
    // Write your code here
    if (score > 25 && score <= 30) {
      grade = "A";
    } else if (score > 20 && score <= 25) {
      grade = "B";
    } else if (score > 15 && score <= 20) {
      grade = "C";
    } else if (score > 10 && score <= 15) {
      grade = "D";
    } else if (score > 5 && score <= 10) {
      grade = "E";
    } else {
      grade = "F";
    }
  
    return grade;
  }
  
  function main() {
    const score = +readLine();
  
    console.log(getGrade(score));
  }
  