import java.awt.geom.Point2D;
import java.lang.reflect.Array;
import java.math.BigInteger;
import java.util.*;

public class Main {	
	Scanner in = new Scanner(System.in);
	public static void main(String[] args){
		new Main();
	}
	public Main(){		
		
		new AOJI_A();
		
	}
	
	class AOJI_A{
		public AOJI_A() {
			while(true){
				int a = in.nextInt();
				int b = in.nextInt();
				int c = in.nextInt();
				if(a+b+c==-3)break;
				if(a==-1||b==-1)System.out.println("F");
				else if(a+b>=80)System.out.println("A");
				else if(a+b>=65)System.out.println("B");
				else if(a+b>=50)System.out.println("C");
				else if(a+b>=30){
					if(c>=50)System.out.println("C");
					else System.out.println("D");
				}else System.out.println("F");
			}
		}
	}
	
	class AOJ0184{
		public AOJ0184() {
			while(true){
				int n=in.nextInt();
				if(n==0)break;
				int[] ans = new int[7];
				for(int i=0;i<n;i++){
					int num = in.nextInt();
					if(num==0)ans[0]++;
					else if (num<60)ans[num/10]++;
					else ans[6]++;
				}
				for(int i=0;i<7;i++)System.out.println(ans[i]);
			}
		}
	}
	
	class AOJ0097{
		public AOJ0097() {//ナップザック問題
			while(true){
				int n=in.nextInt();
				int s=in.nextInt();
				if(n+s==0)break;
				
				
				
				
				
				
			}
			
			
			
			
			
			
			
		}
	}
	
	class AOJ2503{
		int MAX = 0;
		public AOJ2503() {
			Scanner in = new Scanner(System.in);
			int n = in.nextInt();
			int m = in.nextInt();

			int[][] cost = new int[n][n];

			int[] dp = new int[n];
			for(int i=0;i<n*n;i++){
				cost[i/n][i%n]=MAX;
			}
			for(int i=0;i<m;i++){
				int a = in.nextInt();
				int b = in.nextInt();
				cost[a][b]=in.nextInt();
			}	
			//			TODO 一番大きいパスを0のポイントからn-1のポイントまでの

			for(int s=1;s<n;s++){
				for(int i=0;i<=s;i++){
					if(cost[i][s]==MAX)continue;
					dp[s]=Math.max(dp[s],dp[i]+cost[i][s]);
				}
			}
			for(int s=0;s<n;s++)for(int i=0;i<n;i++){
				if(cost[s][i]==MAX)continue;
				dp[i]=Math.max(dp[s]+cost[s][i],dp[i]);
			}
			for(int i=0;i<n;i++)System.out.print(dp[i]+" ");
			System.out.println();
			System.out.println(dp[n-1]);
		}
	}



	class  AOJ0155{
		int n;
		Building[] build;
		double[][] cost;
		double Max;
		int[] list;
		void dijkstra(int s,int g){
			list = new int[n];
			double[] d = new double[n];
			boolean used[] = new boolean[n];
			Arrays.fill(d,Max);
			Arrays.fill(used, false);
			d[s] = 0;
			int v=-1;
			while(true){
				int before = v;
				v=-1;
				for(int u=0;u<n;u++)if(!used[u]&&(v==-1||d[u]<d[v]))v=u;//まだ使われていない頂点のうち最小のものを探す
				if(v==-1)break;
				System.out.println(v);
				list[v] = before;//前の頂点を記憶
				if(v==g)break;
				used[v]=true;//行き先は頂点v
				for(int u=0;u<n;u++){
					d[u] = Math.min(d[u],d[v]+cost[v][u]);
				}
			}
		}
		ArrayList<Integer> list2;
		
		void bfs(int g){
			System.out.println(g);
			list2.add(build[g].ban);
			if(list[g]==-1)return;
			bfs(list[g]);
			return;
		}
		
		public AOJ0155() {
			Max = Double.MAX_VALUE;
			while(true){
				n = in.nextInt();
				if(n==0)break;
				cost = new double[n][n];
				build = new Building[n];
				for(int i=0;i<n;i++){
					int ban = in.nextInt();
					int x = in.nextInt();
					int y = in.nextInt();
					build[i] = new Building(x, y, ban);
				}
				
				for(int s=0;s<n;s++){
					for(int i=0;i<n;i++){
						cost[s][i] =  Math.hypot(build[s].x-build[i].x, build[s].y-build[i].y);
						cost[s][i] = cost[s][i]>50? Max:cost[s][i]; 
						cost[i][s]=cost[s][i];
					}
				}
				
				//costデバック
//				double rei = 0;
//				for(int i=0;i<n;i++){
//					for(int s=0;s<n;s++)if(cost[i][s]!=Max)System.out.printf("%5f ",cost[i][s]);
//					else System.out.printf("%5f",rei);
//					System.out.println();
//				}
				//ここまでデバック
				
				int k = in.nextInt();
				for(int i=0;i<k;i++){
					int from = in.nextInt();
					int goal = in.nextInt();
					dijkstra(from-1,goal);
					list2 = new ArrayList<Integer>();
					for(int s=0;s<n;s++)System.out.print(list[s]+" before    ");
					System.out.println();
					bfs(goal-1);
					for(int s=list2.size()-1;s>0;s--){
						System.out.print(list2.get(s)+" ");
					}
					System.out.println(list2.get(0));
				}
			}
		}
		
		class Building{
			int x,y,ban;
			public Building(int x,int y,int ban) {
				this.x = x;
				this.y = y;
				this.ban = ban;
			}
		}
		
	}
}

Measured memory_bytes: 19272