#include<bits/stdc++.h> using namespace std; #define ll long long #define pll pair<ll,ll> #define mll map<ll,ll> int main(){ ll a; cin>>a; char c; if(a>=90)c='A'; else if(a>=80)c='B'; else if(a>=70)c='C'; else if(a>=60)c='D'; else c='F'; cout<<c; return 0; }