Problem2197--ABC159A - The Number of Even Pairs

2197: ABC159A - The Number of Even Pairs

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Time Limit : 1.000 sec  Memory Limit : 128 MB

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Description

Problem Statement

We have N+MN+M balls, each of which has an integer written on it.
It is known that:
  • The numbers written on NN of the balls are even.
  • The numbers written on MM of the balls are odd.
Find the number of ways to choose two of the N+MN+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 1000≤N,M≤100
  • 2 \leq N+M2≤N+M
  • All values in input are integers.

Input

Input is given from Standard Input in the following format: 
NNMM

Output

Print the answer.

Sample Input Copy 

2 1

Sample Output Copy 

1

HINT

For example, let us assume that the numbers written on the three balls are 1,2,41,2,4.

  • If we choose the two balls with 11 and 22, the sum is odd;
  • If we choose the two balls with 11 and 44, the sum is odd;
  • If we choose the two balls with 22 and 44, the sum is even.

Thus, the answer is 11.

Source/Category

AtCoder 
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