#include <stdio.h>
 int main()
{
   int mark[4];
   int i,sum=0;
   for 	(i=0;i<4;i++)
   {
   	scanf ("%d",&mark[i]); 
   	if (mark[i]>=90) 
   	{
   		sum+=1;
	}
   }
   switch (sum)
   {
   	case 4:printf("%s","A");break;
   	case 3:
   	case 2:
	   printf("%s","B");break;
	case 1:printf("%s","C");break;
	case 0:printf("%s","D");break;
   }
   return 0;
} 
/**************************************************************
	Problem: 1735
	User: admin
	Language: C
	Result: Accepted
	Time:13 ms
	Memory:1144 kb
****************************************************************/