http://arc003.contest.atcoder.jp/tasks/arc003_1

arc003_1.js

process.stdin.resume();
process.stdin.setEncoding('utf8');
process.stdin.on('data', function (chunk) {
	var lines = chunk.split('\n');
	var len = parseInt(lines.shift(),10);
	var result = (function(grades) {
		var score_table = {
			'A':4,'B':3,'C':2,'D':1,'F':0
		};

		var sum =  grades.map(function(v) {
			return score_table[v];
		})
		.reduce(function(prev,current) {
			return prev + current;
		});
		
		return  sum / grades.length;

	}(lines[0].toString().split('').slice(0,len)));

	console.log(result);
});