Problem:
Score : 100 points

Problem Statement

We have N+M balls, each of which has an integer written on it.

It is known that:

The numbers written on N of the balls are even.

The numbers written on M of the balls are odd.

Find the number of ways to choose two of the N+M balls (disregarding order) so that the sum of the numbers written on them is even.

It can be shown that this count does not depend on the actual values written on the balls.

Constraints

0 \leq N,M \leq 100

2 \leq N+M

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M

Output

Print the answer.

Sample Input 1

2 1

Sample Output 1

1

For example, let us assume that the numbers written on the three balls are 1,2,4.

If we choose the two balls with 1 and 2, the sum is odd;

If we choose the two balls with 1 and 4, the sum is odd;

If we choose the two balls with 2 and 4, the sum is even.

Thus, the answer is 1.

Sample Input 2

4 3

Sample Output 2

9

Sample Input 3

1 1

Sample Output 3

0

Sample Input 4

13 3

Sample Output 4

81

Sample Input 5

0 3

Sample Output 5

3

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace shoboi_program
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            string[] inputStrArray = Console.ReadLine().Split(' ');

            Int32 nnn = 0;
            Int32.TryParse(inputStrArray[0], out nnn);
            Int32 mmm = 0;
            Int32.TryParse(inputStrArray[1], out mmm);

            // N個の偶数ボール　// M個の奇数ボールがある
            // 2つ選んで足したら偶数になるのは何通り？

            // 偶数2個 or 奇数2個の組み合わせを求める
            Int32 xCombi = (nnn * (nnn - 1))/2;
            Int32 yCombi = (mmm * (mmm - 1))/2;

            Console.Write("{0}", xCombi + yCombi); // 文字や数値の出力
        }
    }
}