Problem:
Score : 100 points

Problem Statement

Find the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive). The order does not matter.

Constraints

2\leq K\leq 100

K is an integer.

Input

Input is given from Standard Input in the following format:

K

Output

Print the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive).

Sample Input 1

3

Sample Output 1

2

Two pairs can be chosen: (2,1) and (2,3).

Sample Input 2

6

Sample Output 2

9

Sample Input 3

11

Sample Output 3

30

Sample Input 4

50

Sample Output 4

625

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.*;
import java.util.*;


public class Main {

    class A {
        int a;
        int b;

        public A(int a, int b) {
            this.a = a;
            this.b = b;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            A a1 = (A) o;
            return a == a1.a &&
                    b == a1.b;
        }

        @Override
        public int hashCode() {
            return Objects.hash(a, b);
        }
    }


    private void solve(InputReader in, PrintWriter out) {
        int n = in.nextInt();
        List<Integer> list = new ArrayList<>();
        List<Integer> list2 = new ArrayList<>();
        Set<A> set = new HashSet<>();
        for (int i = 1; i <= n; i++) {
            if (i % 2 == 0) {
                list.add(i);
            } else {
                list2.add(i);
            }
        }
        for (int i = 0; i < list.size(); i++) {
            for (int j = 0; j < list2.size(); j++) {
                set.add(new A(list.get(i), list2.get(j)));
            }
        }
        out.println(set.size());
    }


    public static void main(String[] args) {
        new Main().run();
    }

    private void run() {
        try (
                InputReader in = new InputReader(System.in);
                PrintWriter out = new PrintWriter(System.out)) {
            solve(in, out);
        }
    }


    static class InputReader implements AutoCloseable {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }


        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        @Override
        public void close() {

        }
    }

}