const fs= require('fs')
const inputData = fs.readFileSync('/dev/stdin')
const a = inputData
if (100>=a && a>=90){
console.log('A')
}
else if(89>=a && a>=80){
console.log('B')
}
else if(79>=a && a>=70){
console.log('C')
}
else if(69>=a && a>=60){
console.log('D')
}
else{
console.log('F')
}