import java.util.*;

public class Main {
    private static double solve(int N, int[] a) {
        return 0;
    }

    public static double run(Scanner scanner) {
        int N = scanner.nextInt();
        int[] a = new int[N];
        for (int i=0; i < N; i++) a[i] = scanner.nextInt();

        return solve(N, a);
    }

    public static void main(String[] args) {
        try (Scanner scanner = new Scanner(System.in)) {
            System.out.println(run(scanner));
        }
        Tests.run();
    }
}

class Tests {
    public static void run() {
        testCase("3\n" +
                "1 1 1", 5.5);
        testCase("1\n" +
                "3", 3);
        testCase("2\n" +
                "1 2", 4.5);
        testCase("10\n" +
                "1 3 2 3 3 2 3 2 1 3", 54.48064457488221);

        System.out.println("DONE");
    }

    private static void testCase(String input, double expected) {
        try (Scanner scanner = new Scanner(input)) {
            double result = Main.run(scanner);
            if (Math.abs(result-expected) > 1e-9) System.out.println("TEST FAILED: was " + result + " expected " + expected + " on input\n" + input);
        }
    }
}