就第二题过了。。。

public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int w = in.nextInt();
        int len = n * 2;
        double[] a = new double[len];
        for (int i = 0; i < len; i++) {
            a[i] = in.nextDouble();
        }
        Arrays.sort(a);
        double x = a[n];
        double y = a[0];
        double result = 0;
        if(x>=y*2) {
            result = 3*n*y;
        }else {
            result = (x/2.0*n)+x*n;
        }
        result = Math.min(result, w);
        System.out.printf("%.6f\n",result);
    }