var grade = (marks)=>{
		const avg = calculateAverage(marks);
		if (avg < 59) return "F";
		if (avg < 69) return "D";
		if (avg < 79) return "C";
		if (avg < 89) return "B";
		return "A";

}
var calculateAverage=(para)=>{
	var sum = 0;
	for (var value =1; value <=para; value++)
	var sum = sum + value;
    return  sum /para.length;


}

console.log(grade(56));