#include<bits/stdc++.h> using namespace std; int main() { int a; cin>>a; if(a>=90&&a<100)cout<<'A'<<endl; else if(a>=80&&a<90)cout<<'B'<<endl; else if(a>=70&&a<80)cout<<'C'<<endl; else if(a>=60&&a<70)cout<<'D'<<endl; else cout<<'F'<<endl; return 0; }