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

public class Solve
{
    static void Main(string[] args)
    {
        new ABC_A();
    }
}
public class ABC_A
{
    public ABC_A()
    {
        var inn=Ex.ReadIntArray();
        int n = inn[0];
        int m = inn[1];
        Console.WriteLine(n*(n-1)/2+m*(m-1)/2);
    }
}
public class ABC_B
{
    public ABC_B()
    {

    }
}
public class ABC_C
{
    public ABC_C()
    {

    }
}
public class ABC_D
{
    public ABC_D()
    {

    }
}
public class ABC_E
{
    public ABC_E()
    {

    }
}
public class ABC_F
{
    public ABC_F()
    {

    }
}
public static class Ex
{
    public static string Read { get { return Console.ReadLine().Trim(); } }
    public static int ReadInt { get { return int.Parse(Read); } }
    public static long ReadLong { get { return long.Parse(Read); } }
    public static double ReadDouble { get { return double.Parse(Read); } }
    public static string[] ReadArray() { return Read.Split(' '); }
    public static int[] ReadIntArray() { return Read.Split(' ').Select(s => int.Parse(s)).ToArray(); }
    public static long[] ReadLongArray() { return Read.Split(' ').Select(s => long.Parse(s)).ToArray(); }
    public static string[] ReadArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
    public static int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
    public static long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
}