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++14 (GCC 5.4.1) (judge verdict: Accepted)

#pragma comment(linker,"/stack:16777216")
#include<iostream>
#include<cstdio>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<list>
#include<stack>
#include<queue>
#include<deque>
#include<functional>
#include<utility>
#include<algorithm>
#include<sstream>
#include<cstring>
#include<cmath>
#include<cctype>
#include<exception>
#include<climits>
#include<cfloat>
#include<iomanip>
#include<bitset>
#include<streambuf>
#include<cstdlib>
#include<ctime>
//#include"functions.h"
//#include"highprecision.h"
#define pb push_back
#define pf push_front
#define ppb pop_back
#define mp make_pair
#define np next_permutation
#define fi first
#define sc second
#define vi vector<int>
#define vll vector<long long>
#define vvi vector<vector<int> >
#define vpi vector<pair<int,int> >
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pff pair<float,float>
#define pdd pair<double,double>
#define psi pair<string,int>
#define pis pair<int,string>
#define ppi pair<pii,int>
#define pdi pair<pii,pii>
#define LN 84757
#define INF 1e9
#define LLINF 1e18
#define MAXINT 0x7fffffff
#define MAXLL 0x7fffffffffffffff
#define EPS 1e-5
#define DEPS 1e-9
#define LDEPS 1e-13
#define MOD 1000000007
#define E 2.7182818284
#define PI 3.1415926535
#define rep(a,b) for(int a=0;a<b;a++)
#define rep(a,b) for(int a=0;a<b;a++)
#define forn(a,n) for(int a=1;a<=n;a++)
#define forr(a,n) for(int a=n;a>=1;a--)
#define repn(a,n,p) for(int a=0;a<n;a+=p)
#define repr(a,n) for(int a=n-1;a>=0;a--)
#define foreach(a,b) for(__typeof(b.begin())a=b.begin();a!=b.end();a++)
#define fillemp(a) memset(a,0,sizeof(a))
#define fillinf(a) memset(a,0x3f,sizeof(a))
#define frestream freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
#define stdf(a,b) freopen(a,"r",stdin);freopen(b,"w",stdout);
#define inputf(a) freopen(a,"r",stdin);
#define outputf(a) freopen(a,"w",stdout);
#define sz(a) a.size()
#define all(a) a.begin(),a.end()
#define MOD 1000000007
typedef long long ll;
typedef long double ld;
using namespace std;

int main(int argc,char** argv){
	int n,m;
	cin>>n>>m;
	cout<<n*(n-1)/2+m*(m-1)/2;
	return 0;
}