#include <iostream>
using namespace std;
int main() {
int n;
cin >> n;
if(n >= 90)
cout << "A" << endl;
else if(n >= 80)
cout << "B" << endl;
else if(n >= 70)
cout << "C" << endl;
else if(n >= 60)
cout << "D" << endl;
else
cout << "F" << endl;
return 0;
}
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