Problem:
Score : 200 points

Problem Statement

We have a board with H horizontal rows and W vertical columns of squares.
There is a bishop at the top-left square on this board.
How many squares can this bishop reach by zero or more movements?

Here the bishop can only move diagonally.
More formally, the bishop can move from the square at the r_1-th row (from the top) and the c_1-th column (from the left) to the square at the r_2-th row and the c_2-th column if and only if exactly one of the following holds:

r_1 + c_1 = r_2 + c_2

r_1 - c_1 = r_2 - c_2

For example, in the following figure, the bishop can move to any of the red squares in one move:

Constraints

1 \leq H, W \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

H \ W

Output

Print the number of squares the bishop can reach.

Sample Input 1

4 5

Sample Output 1

10

The bishop can reach the cyan squares in the following figure:

Sample Input 2

7 3

Sample Output 2

11

The bishop can reach the cyan squares in the following figure:

Sample Input 3

1000000000 1000000000

Sample Output 3

500000000000000000

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;

namespace Panasonic2020B
{
    class Input
    {
        /// <summary>
        /// 1行の入力を取得する
        /// </summary>
        /// <returns>文字列</returns>
        public void String(out string s)
        {
            s = Console.ReadLine();
        }

        /// <summary>
        /// 複数行の入力を取得
        /// </summary>
        /// <returns>文字列の配列</returns>
        public void String(long rowNumber, out string[] s)
        {
            s = new String[rowNumber];
            for (long i = 0; i < rowNumber; i++)
            {
                String(out s[i]);
            }
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>int型の値</returns>
        public void Int(out int i)
        {
            string s;
            String(out s);
            i = int.Parse(s);
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <param name="a"></param>
        public void Long(out long a)
        {
            string s;
            String(out s);
            a = long.Parse(s);
        }

        /// <summary>
        /// 2つの整数が1行に書かれている入力を、2つのlongで受け取る
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        public void Long(out long a, out long b)
        {
            long[] lArr;
            Long(out lArr);
            a = lArr[0];
            b = lArr[1];
        }

        /// <summary>
        /// 3つの整数が1行に書かれている入力を、3つのlongで受け取る
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <param name="c"></param>
        public void Long(out long a, out long b, out long c)
        {
            long[] lArr;
            Long(out lArr);
            a = lArr[0];
            b = lArr[1];
            c = lArr[2];
        }

        /// <summary>
        /// 4つの整数が1行に書かれている入力を、4つのlongで受け取る
        /// </summary>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <param name="c"></param>
        /// <param name="d"></param>
        public void Long(out long a, out long b, out long c, out long d)
        {
            long[] lArr;
            Long(out lArr);
            a = lArr[0];
            b = lArr[1];
            c = lArr[2];
            d = lArr[3];
        }

        public void Long(out long[] lArr)
        {
            lArr = Split().Select(long.Parse).ToArray();
        }

        public void Long(long rowNumber, out long[] lArr)
        {
            lArr = new long[rowNumber];
            for (long i = 0; i < rowNumber; i++)
            {
                Long(out lArr[i]);
            }
        }

        /// <summary>
        /// 2つの整数が複数行に書かれている入力を、2つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="a"></param>
        /// <param name="b"></param>
        public void Long(long rowNumber, out long[] a, out long[] b)
        {
            a = new long[rowNumber];
            b = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                Long(out a[i], out b[i]);
            }
        }

        /// <summary>
        /// 3つの整数が複数行に書かれている入力を、3つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <param name="c"></param>
        public void Long(long rowNumber, out long[] a, out long[] b, out long[] c)
        {
            a = new long[rowNumber];
            b = new long[rowNumber];
            c = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                Long(out a[i], out b[i], out c[i]);
            }
        }

        /// <summary>
        /// 4つの整数が複数行に書かれている入力を、4つのlong[]で受け取る
        /// </summary>
        /// <param name="rowNumber"></param>
        /// <param name="a"></param>
        /// <param name="b"></param>
        /// <param name="c"></param>
        /// <param name="d"></param>
        public void Long(long rowNumber, out long[] a, out long[] b, out long[] c, out long[] d)
        {
            a = new long[rowNumber];
            b = new long[rowNumber];
            c = new long[rowNumber];
            d = new long[rowNumber];
            for (int i = 0; i < rowNumber; i++)
            {
                Long(out a[i], out b[i], out c[i], out d[i]);
            }
        }

        /// <summary>
        /// h行w列のLongのmapを入力する
        /// </summary>
        /// <param name="h"></param>
        /// <param name="w"></param>
        /// <param name="a"></param>
        public void Long(long h, long w, out long[][] a)
        {
            a = new long[h][];
            for (long i = 0; i < h; i++)
            {
                Long(out a[i]);
            }
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>double型の値</returns>
        public void Double(out double d)
        {
            string s;
            String(out s);
            d = double.Parse(s);
        }

        /// <summary>
        /// 1行の入力を取得
        /// </summary>
        /// <returns>double型の配列</returns>
        public void Double(out double[] dArr)
        {
            dArr = Split().Select(double.Parse).ToArray();
        }

        private IEnumerable<string> Split()
        {
            string s;
            String(out s);
            return s.Split(' ');
        }
    }

    class Program
    {
        public static void Main(string[] args)
        {
            Solver solver = new Solver();
            solver.Solve();
        }
    }

    class Solver
    {
        private Input input;

        public Solver()
        {
            input = new Input();
            input.Long(out H, out W);
        }

        private long H;
        private long W;

        public void Solve()
        {
            Console.WriteLine(H * W / 2 + (H * W % 2));
        }
    }
}