function Main(input) {
    input = input.trim().split("\n").map(function(x) { return x.split(" ")});
    var N = parseInt(input[0], 10);
    var ary_xy = [];
    var sumWay = 0;
    var xDiff, yDiff, ans;
    for (var i = 1; i <= N; i++){
    	ary_xy.push(input[i].map(e => parseInt(e, 10)));
    }
    for (var i = 0; i < N; i++){
    	for (var j = i + 1; j < N; j++){
    		xDiff = ary_xy[j][0] - ary_xy[i][0];
    		yDiff = ary_xy[j][1] - ary_xy[i][1];
    		sumWay += Math.sqrt(Math.pow(xDiff, 2) + Math.pow(yDiff, 2));	
    	}
    }
    ans = sumWay * 2 / N;
	console.log(ans);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));