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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>
#include <stdlib.h>
#include <string.h>

typedef unsigned long long int uint64;
typedef long long int          int64;
 
#define MAX(a,b) ((a)>(b)?(a):(b))
#define MIN(a,b) ((a)<(b)?(a):(b))
#define ABS(a) ((a)>(0)?(a):-(a))

//set many funcs template
//Ver.20181228
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<stdbool.h>
#include<time.h>

void get_N (int64 *n)
{
  int cnt;
  
  cnt = scanf("%lld", n);

  return;
}

void get_NK (int64 *n, int64 *k)
{
  int cnt;
  
  cnt = scanf("%lld %lld", n, k);

  return;
}

void get_N1234 (int64 *n1, int64 *n2, int64 *n3, int64 *n4)
{
  int cnt;

  if (n2 == NULL) {
    cnt = scanf("%lld", n1);
    return;
  }
  if (n3 == NULL) {
    cnt = scanf("%lld %lld", n1, n2);
    return;
  }
  if (n4 == NULL) {
    cnt = scanf("%lld %lld %lld", n1, n2, n3);
    return;
  }
  cnt = scanf("%lld %lld %lld %lld", n1, n2, n3, n4);
  return;
}

void omit_LF (void)
{
  char c;
  int  cnt;
  
  cnt = scanf("%c", &c);
  return;
}

char *get_S (int64 n)
{
  char *buf;
  int  i;
  int  cnt;

  buf = malloc((size_t)(n * sizeof(char)));
  if (buf == NULL) {
    return NULL;
  }

  cnt = scanf("%s", buf);

  return buf;
}

char *get_SN (int64 max, int64 *size)
{
  char *buf;
  int  i;
  int  n;
  int  cnt;

  buf = malloc((size_t)(max * sizeof(char)));
  if (buf == NULL) {
    return NULL;
  }

  cnt = scanf("%s", buf);

  n = strlen(buf);

  *size = n;
  
  return buf;
}

int64 *get_NSeq (int64 n)
{
  int64 *buf;
  int64 i;

  buf = malloc((size_t)(n * sizeof(int64)));
  for (i=0; i<n; i++) {
    get_N(&buf[i]);
  }

  return buf;
}

typedef struct _tmp_t {
  int64    a;
  int64    b;
} tmp_t;


int compare_a(const void *a, const void *b)
{
  tmp_t *x;
  tmp_t *y;

  x = (tmp_t *)a;
  y = (tmp_t *)b;

  if (x->a > y->a) return 1;
  if (x->a < y->a) return -1;

  return 0;
}

int compare_b(const void *a, const void *b)
{
  tmp_t *x;
  tmp_t *y;

  x = (tmp_t *)a;
  y = (tmp_t *)b;

  if (x->b > y->b) return 1;
  if (x->b < y->b) return -1;

  return 0;
}

void qsort_a (tmp_t *array, int size)
{
  qsort(array, size, sizeof(tmp_t), compare_a);
}

void qsort_b (tmp_t *array, int size)
{
  qsort(array, size, sizeof(tmp_t), compare_b);
}

long long llnCr(long long a,long long b){long long i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}


int main()
{
  int64 N,M;
  int64 c;

  get_NK(&N, &M);

  c = llnCr(N, 2);
  c += llnCr(M, 2);

  printf("%lld", c);
  return 0;
}