// editor4
#include<iostream>
using namespace std;
int main()
{
    int  A,B,C,D;
    cin>>A>>B>>C>>D>>F;
    if(A<=90&&A<=100)
    {
        cout<<"A"<<endl;
    }
    else if(B<=80&&B<=89)
    {
        cout<<"B"<<endl;
    }
    else if(C<=70&&C<=79)
    {
        cout<<"C"<<endl;
    }
    else if(D<=60&&D<=69)
    {
        cout<<"D"<<endl;
    }
    else if(F<60)
    {
        cout<<"F"<<endl;
    }
    else 
    {
        cout<<"Ivalid input";
    }
}