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 Ruby (2.3.3) (judge verdict: Accepted)

#常に2なので定数宣言
T = 2
T.freeze

#値取得
n, m = gets.chomp.split.map(&:to_i)

=begin
答えが偶数になる場合は以下の通りしかない
偶数＋偶数 ＝ 偶数
奇数＋奇数 ＝ 奇数
つまりこれらの組み合わせ（重複なし）を求める
=end

#便宜的に配列生成
aryk = Array.new(m, "foo")
aryg = Array.new(n, "foo")

#奇数+奇数が成立する場合
wk1 = aryk.combination(T).to_a.count # n_C_k

#偶数+偶数が成立する場合
wk2 = aryg.combination(T).to_a.count # n_C_k

ans = wk1.to_i + wk2.to_i
puts ans