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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.*;
import java.io.*;

//294784RY

public class Main {
    public static PrintStream out = new PrintStream(System.out);
    static LinkedList<Integer> adj[];
    static boolean[] vis;
    static final int nINF = Integer.MIN_VALUE;
    static final int INF = Integer.MAX_VALUE;
    public static void main(String[] args) {
        FastScanner sc = new FastScanner();
        int l = sc.nextInt();
        //double x = l / 3;
        out.printf("%.12f", Math.pow(l, 3) / 27);
        out.println();
        
//        int n = sc.nextInt();
//        HashMap<Integer, Integer> map = new HashMap();
//        int[] arr = new int[n];
//        int[] freq = new int[200005];
//        for(int i = 0; i < n; i++){
//            int x = sc.nextInt();
//            if(!map.containsKey(x))
//                map.put(x, 1);
//            freq[x]++;
//            arr[i] = x;
//        }
//        long sum = 0;
//        //out.println("map size: " + map.size());
//        for(Map.Entry<Integer,Integer> entry : map.entrySet()){
//            int curr = entry.getKey();
//            int x = freq[curr];
//            double a  = x * ( (double) (x - 1) / 2);
//            sum += (int) a;
//        }
//        //out.println("sum: " + sum);
//        for(int i = 0; i < n; i++){
//            int curr  = arr[i];
//            int oldFreq = freq[curr];
//            double a  = oldFreq * ( (double) (oldFreq - 1) / 2);
//            int newFreq = freq[curr] - 1;
//            double b  = newFreq * ( (double) (newFreq - 1) / 2);
//            long sub = (long) a - (long) b;
//            //out.println("sub: " + sub);
//            out.println(sum - sub);
//        }
    }
    
    static int p(int x, int a){
        int res = 1; 
        while (a>0) 
        { 
            if ((a & 1)>0) 
                res = res * x; 
            x = x * x; 
            a >>= 1; 
        } 
        return res;        
    }
    
    static void initAdj(int n){
        adj = new LinkedList[n + 2];
        for(int i = 0; i <= n + 1; i++)
            adj[i] = new LinkedList();
    }    
    
    static void dfs(int n){
        Iterator<Integer> i = adj[n].listIterator();
        while(i.hasNext()){
            Integer a = i.next();
            if(!vis[a]){
                vis[a] = true;
                dfs(a);
            }
        }
    }
    
    static ArrayList<Integer> sieve(int n){
        boolean prime[] = new boolean[n+1]; 
        ArrayList<Integer> res = new ArrayList();
        for(int i=0;i<n;i++) 
            prime[i] = true; 
          
        for(int p = 2; p*p <=n; p++) 
        { 
            // If prime[p] is not changed, then it is a prime 
            if(prime[p] == true) 
            { 
                // Update all multiples of p 
                for(int i = p*p; i <= n; i += p) 
                    prime[i] = false; 
            } 
        } 

        for(int i = 2; i <= n; i++) 
        { 
            if(prime[i] == true) 
                res.add(i);
        }
        return res;
    }
    
    //returns index; outputs earliest occurence; -1 if target value does not exist
    static int getUpper(int[] arr, int target)  
    {  
        int start = 0, end = arr.length - 1;  
    
        int ans = -1;  
        while (start <= end) {  
            int mid = (start + end) / 2;  
    
            // Move to right side if target is  
            // greater.  
            if (arr[mid] <= target) {  
                start = mid + 1;  
            }  
    
            // Move left side.  
            else {  
                ans = mid;  
                end = mid - 1;  
            }  
        }  
        return ans;  
    }  
    
    //returns index; outputs latest occurence; -1 if target value does not exist
    static int getLower(int[] arr, int target)  
    {  
        int start = 0, end = arr.length-1;  
    
        int ans = -1;  
        while (start <= end) {  
            int mid = (start + end) / 2;  
    
            // Move to the left side if the target is smaller  
            if (arr[mid] >= target) {  
                end = mid - 1;  
            }  
    
            // Move right side  
            else {  
                ans = mid;  
                start = mid + 1;  
            }  
        }  
        return ans;  
    }  
     
   

    static String removeLastChar(String ss) {
        if (ss == null || ss.length() == 0) {
            return ss;
        }
        return ss.substring(0, ss.length()-1);
    }
    
    static void solveLunLun(){
        FastScanner sc = new FastScanner();
        Queue<Long> q = new ArrayDeque();
        for(int i = 1; i <= 9; i++){
            q.add((long) i);
        }
        int k = sc.nextInt();
        int cnt = 0;
        long x = 1;
        while(cnt < k){
            x = q.poll();
            cnt++;
            if(cnt >= k) break;
            
            long d = x % (long) 10;
            if(d == 0){
                q.add( (10 * x));
                q.add( (10 * x) + d + 1 );
            }
            else if (d == 9){
                q.add( (10 * x) + d - 1);
                q.add( (10 * x) + d);
            }
            else {
                q.add( (10 * x) + d - 1);
                q.add( (10 * x) + d);
                q.add( (10 * x) + d + 1 );                
            }
        }
        out.println(x);

    }
    
    private static int factorial(int n){
            int fac = 1;
            for(int i=2;i<=n;i++){
                    fac *= i;
            }
 
            return fac;
    }
    
    private static boolean isPrime(int n){
            if(n == 0 || n == 1){
                    return false;
            }else if(n%2 == 0){
                    return false;
            }else{
                    int x = (int)(Math.sqrt(n)) + 1;
                    for(int i=3;i<=x;i+=2){
                            if(n % i == 0){
                                    return false;
                            }
                    }
                    return true;
            }
    }
 
    private static long pow(int base,int exp){
            if(exp == 0){
                    return 1;
            }else if(exp == 1){
                    return base;
            }else{
                    long a = pow(base,exp/2);
                    if(exp%2 == 0){
                            return a * a;
                    }else{
                            return a * a * base;
                    }
            }
 
    }
 
    private static int gcd(int a,int b){
            if(a == 0){
                    return b;
            }
 
            return gcd(b%a,a);
    }
 
    private static int lcm(int a,int b){
            return (a * b)/gcd(a,b);
    }
    

    
    private static int BSearch(int target, int[] arr){
        int index = -1;
        int l = 0;
        int r = arr.length - 1;
        
        while(l <= r){
            int mid = (l + r) / 2;
            if(arr[mid] <= target){
                index = mid;
                l = mid + 1;
            } else{
                r = mid - 1;    
            }
        }
        
        return index;
    }
        
 
}


class FastScanner { 
    BufferedReader br; 
    StringTokenizer st; 

    public FastScanner() 
    { 
        br = new BufferedReader(new
                 InputStreamReader(System.in)); 
    } 

    String next() 
    { 
        while (st == null || !st.hasMoreElements()) 
        { 
            try
            { 
                st = new StringTokenizer(br.readLine()); 
            } 
            catch (IOException  e) 
            { 
                e.printStackTrace(); 
            } 
        } 
        return st.nextToken(); 
    } 

    int nextInt() 
    { 
        return Integer.parseInt(next()); 
    } 

    long nextLong() 
    { 
        return Long.parseLong(next()); 
    } 

    double nextDouble() 
    { 
        return Double.parseDouble(next()); 
    } 

    String nextLine() 
    { 
        String str = ""; 
        try
        { 
            str = br.readLine(); 
        } 
        catch (IOException e) 
        { 
            e.printStackTrace(); 
        } 
        return str; 
    } 

    int[] nextIntArray(int length) {
       int[] arr = new int[length];
       for (int i = 0; i < length; i++)
           arr[i] = nextInt();
       return arr;
   }
} 

class Node
{
    public int V;
    public Node(int v)
    {
        this.V=v;
    }
}



//    private static ArrayList<Integer> primeSieve(int n){
//        boolean b[] = new boolean[n+1];
//        b[0] = true;
//        b[1] = true;
//        for(int i=2;i<=5;i++){
//                int j = 2;
//                while(i * j <= n && !b[i]){
//                        b[i * j] = true;
//                        j++;
//                }
//        }
// 
//        ArrayList<Integer> arr = new ArrayList<>();
//        for(int i=2;i<n+1;i++){
//                if(!b[i]){
//                        arr.add(i);
//                }
//        }
// 
//        return arr;
//    }