import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
	public static void main(String[] args) throws Exception {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	}
} Main public static void main(String[] args) throws Exception {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	} main {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	} BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    } true {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    } String[] line = br.readLine().split(" "); line br.readLine().split(" ") br.readLine().split br.readLine() br.readLine br " " int m = Integer.parseInt(line[0]); m Integer.parseInt(line[0]) Integer.parseInt Integer line[0] line 0 int f = Integer.parseInt(line[1]); f Integer.parseInt(line[1]) Integer.parseInt Integer line[1] line 1 int r = Integer.parseInt(line[2]); r Integer.parseInt(line[2]) Integer.parseInt Integer line[2] line 2 if (m == -1 && f == -1 && r == -1) break; m == -1 && f == -1 && r == -1 m == -1 && f == -1 && r == -1 m == -1 m -1 1 f == -1 f -1 1 r == -1 r -1 1 break; if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    } m == -1 || f == -1 m == -1 m -1 1 f == -1 f -1 1 {
		    	System.out.println("F");
		    } System.out.println("F") System.out.println System.out System System.out "F" if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    } 80 <= m + f 80 m + f m f {
		    	System.out.println("A");
		    } System.out.println("A") System.out.println System.out System System.out "A" if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    } 65 <= m + f && m + f < 80 65 <= m + f 65 m + f m f m + f < 80 m + f m f 80 {
		    	System.out.println("B");
		    } System.out.println("B") System.out.println System.out System System.out "B" if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    } 50 <= m + f && m + f < 65 50 <= m + f 50 m + f m f m + f < 65 m + f m f 65 {
		    	System.out.println("C");
		    } System.out.println("C") System.out.println System.out System System.out "C" if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    } 30 <= m + f && m + f < 50 30 <= m + f 30 m + f m f m + f < 50 m + f m f 50 {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	} 50 <= r 50 r {
			    	System.out.println("C");
		    	} System.out.println("C") System.out.println System.out System System.out "C" {
			    	System.out.println("D");
		    	} System.out.println("D") System.out.println System.out System System.out "D" if (m + f < 30) {
		    	System.out.println("F");
		    } m + f < 30 m + f m f 30 {
		    	System.out.println("F");
		    } System.out.println("F") System.out.println System.out System System.out "F" String[] args args public class Main {
	public static void main(String[] args) throws Exception {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	}
} public class Main {
	public static void main(String[] args) throws Exception {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	}
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
	    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));


	    while (true) {
		    String[] line = br.readLine().split(" ");

		    int m = Integer.parseInt(line[0]);
		    int f = Integer.parseInt(line[1]);
		    int r = Integer.parseInt(line[2]);

		    if (m == -1 && f == -1 && r == -1) break;

		    if (m == -1 || f == -1) {
		    	System.out.println("F");
		    } else if (80 <= m + f) {
		    	System.out.println("A");
		    } else if (65 <= m + f && m + f < 80) {
		    	System.out.println("B");
		    } else if (50 <= m + f && m + f < 65) {
		    	System.out.println("C");
		    } else if (30 <= m + f && m + f < 50) {
		    	if (50 <= r) {
			    	System.out.println("C");
		    	} else {
			    	System.out.println("D");
		    	}
		    } else 	if (m + f < 30) {
		    	System.out.println("F");
		    }
	    }
	}
}