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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System;
using System.Collections.Generic;
using System.Linq;

namespace ClassLibrary1
{
    public class ListNode
    {
        public int val;
        public ListNode next;
        public ListNode(int x) { val = x; }
    }


    public class Class1
    {
        //static Dictionary<Tuple<long, long>, long> dic = new Dictionary<Tuple<long, long>, long>();

        static long mod = 1000000007;
        static long[] factorial = new long[1000001];
        static long[] invFactorial = new long[1000001];

        private static void calcFanctorial(long N)
        {
            factorial[0] = 1;
            for (int i = 1; i < N + 1; i++)
            {
                factorial[i] = factorial[i - 1] * i;
                factorial[i] %= mod;
            }
            invFactorial[N] = inv(factorial[N]);
            for (int i = (int)N; i > 0; i--)
            {
                invFactorial[i - 1] = invFactorial[i] * i % mod;
            }

        }

        static long[] mem;


        static void Main(string[] args)
        {
            var N = ReadLongArr();

            var res = N[0] * (N[0] - 1) / 2;
            res += N[1] * (N[1] - 1) / 2;

            Console.WriteLine(res);

        }



        class Tree
        {
            public HashSet<long> conn = new HashSet<long>();
        }


        static long bin(long a, long x, long n)
        {
            long y = a;
            long res = 1;
            while (x > 0)
            {
                if (x % 2 == 1)
                {
                    res = res * y % n;
                }
                x /= 2;
                y = y * y % n;
            }
            return res;
        }

        int cmp(string a, string b)
        {
            for (var i = 0; i < Math.Min(a.Length, b.Length); i++)
                if (a[i] < b[i]) return -1;
                else if (a[i] > b[i]) return 1;
            if (a.Length == b.Length) return 0;
            if (a.Length > b.Length) return 1;
            else return -1;
        }

        private static long nCm(long n, long m)
        {
            if (n < m) return 0;
            if (m < 0) return 0;

            return (factorial[n] * invFactorial[m] % mod) * invFactorial[n - m] % mod;
        }

        private static long nCm2(long n, long m)
        {
            if (n < m) return 0;
            if (m < 0) return 0;

            long res = 1;
            for (long i = n; i > n - m; i--)
            {
                res *= i;
                res %= mod;
            }

            return res * invFactorial[m] % mod;
        }

        public static long pow(long a, long b, long mod)
        {
            a %= mod;

            if (b <= 0) return 1;
            var t = pow(a, b / 2, mod);
            if ((b & 1) == 0) return t * t % mod;
            return t * t % mod * a % mod;
        }

        public static long Lcm(long a, long b)
        {
            return a * b % mod * inv(Gcd(a, b)) % mod;
        }

        public static long Lcm(long[] a)
        {
            List<long> vect = new List<long>();
            long lcm = 1;
            for (int i = 0; i < a.Count(); i++)
            {
                var factor = a[i] / GcdVector(vect, a[i]);
                vect.Add(factor);
                lcm = lcm * factor % mod;
            }
            return lcm;
        }

        static long GcdVector(List<long> a, long b)
        {
            long ma = 1;
            for (int i = 0; i < a.Count; i++)
                ma = ma * a[i] % b;
            return Gcd(b, ma);
        }

        public static long Gcd(long a, long b)
        {
            if (a < b)
                // 引数を入替えて自分を呼び出す
                return Gcd(b, a);
            while (b != 0)
            {
                var remainder = a % b;
                a = b;
                b = remainder;
            }
            return a;
        }

        public static long pow(long a, long b) => pow(a, b, mod);
        public static long inv(long a) => pow(a, mod - 2);

        private static char[][] ReadCharMatrix(long h)
        {
            char[][] mtx = new char[h][];
            for (int i = 0; i < h; i++)
            {
                var s = ReadString();
                mtx[i] = s.ToCharArray();
            }
            return mtx;
        }

        private static int[][] CreateIntMatrix(long h, long w)
        {
            int[][] mtx = new int[h][];
            for (int i = 0; i < h; i++)
            {
                mtx[i] = new int[w];
            }
            return mtx;
        }


        private static string ReadString()
        {
            return Console.ReadLine();
        }
        private static string[] ReadStringArr()
        {
            return Console.ReadLine().Split(' ');
        }
        private static long ReadLong()
        {
            return long.Parse(Console.ReadLine());
        }
        private static long[] ReadLongArr()
        {
            string[] s = ReadStringArr();
            long[] l = new long[s.Length];
            for (int i = 0; i < l.Length; i++)
            {
                l[i] = long.Parse(s[i]);
            }

            return l;
        }
    }
    class StringComparator2 : IComparer<string>
    {
        int IComparer<string>.Compare(string a, string b)
        {
            int length = Math.Min(a.Length, b.Length);
            //Console.WriteLine("Comparing with length {0}", length);
            for (int i = 0; i < length; i++)
            {
                if (a[i] < b[i])
                {
                    //Console.WriteLine("returning -1");
                    return -1;
                }
                else if (a[i] > b[i]) return 1;
            }
            if (a.Length < b.Length) return -1;
            if (a.Length > b.Length) return 1;

            //Console.WriteLine("Why are we at the end?");
            return 0;
        }
    }

    public class PriorityQueue<T> where T : IComparable
    {
        private IComparer<T> _comparer = null;
        private int _type = 0;

        private T[] _heap;
        private int _sz = 0;

        private int _count = 0;

        /// <summary>
        /// Priority Queue with custom comparer
        /// </summary>
        public PriorityQueue(int maxSize, IComparer<T> comparer)
        {
            _heap = new T[maxSize];
            _comparer = comparer;
        }

        /// <summary>
        /// Priority queue
        /// </summary>
        /// <param name="maxSize">max size</param>
        /// <param name="type">0: asc, 1:desc</param>
        public PriorityQueue(int maxSize, int type = 0)
        {
            _heap = new T[maxSize];
            _type = type;
        }

        private int Compare(T x, T y)
        {
            if (_comparer != null) return _comparer.Compare(x, y);
            return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
        }

        public void Push(T x)
        {
            _count++;

            //node number
            var i = _sz++;

            while (i > 0)
            {
                //parent node number
                var p = (i - 1) / 2;

                if (Compare(_heap[p], x) <= 0) break;

                _heap[i] = _heap[p];
                i = p;
            }

            _heap[i] = x;
        }

        public T Pop()
        {
            _count--;

            T ret = _heap[0];
            T x = _heap[--_sz];

            int i = 0;
            while (i * 2 + 1 < _sz)
            {
                //children
                int a = i * 2 + 1;
                int b = i * 2 + 2;

                if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

                if (Compare(_heap[a], x) >= 0) break;

                _heap[i] = _heap[a];
                i = a;
            }

            _heap[i] = x;

            return ret;
        }

        public int Count()
        {
            return _count;
        }

        public T Peek()
        {
            return _heap[0];
        }

        public bool Contains(T x)
        {
            for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
            return false;
        }

        public void Clear()
        {
            while (this.Count() > 0) this.Pop();
        }

        public IEnumerator<T> GetEnumerator()
        {
            var ret = new List<T>();

            while (this.Count() > 0)
            {
                ret.Add(this.Pop());
            }

            foreach (var r in ret)
            {
                this.Push(r);
                yield return r;
            }
        }

        public T[] ToArray()
        {
            T[] array = new T[_sz];
            int i = 0;

            foreach (var r in this)
            {
                array[i++] = r;
            }

            return array;
        }
    }


}