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: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using System.Globalization;
using System.Diagnostics;


class Solve
{
  public Solve() { }
  public static int Main()
  {
    new Solve().calc();
    return 0;
  }


  Scanner cin;

  void calc()
  {
    cin = new Scanner();
    long H = cin.nextLong();
    long W = cin.nextLong();

    if (H == 1 || W == 1)
    {
      Console.WriteLine("1");
      return;
    }
     
    bool dividableBy2 = H % 2 == 0 || W % 2 == 0;
    long result = H * W / 2;
    
    if (!dividableBy2)
    {
      result++;
    }

    Console.WriteLine(result.ToString());
  }
}
class Modulo
{
  private const int M = 1000000007;
  private readonly int[] m_facs;
  public int Mul(int a, int b)
  {
    return (int)(Math.BigMul(a, b) % M);
  }
  public Modulo(int n)
  {
    m_facs = new int[n + 1];
    m_facs[0] = 1;
    for (int i = 1; i <= n; ++i)
    {
      m_facs[i] = Mul(m_facs[i - 1], i);
    }
  }
  public int Fac(int n)
  {
    return m_facs[n];
  }
  public int Pow(int a, int m)
  {
    switch (m)
    {
      case 0:
        return 1;
      case 1:
        return a;
      default:
        int p1 = Pow(a, m / 2);
        int p2 = Mul(p1, p1);
        return ((m % 2) == 0) ? p2 : Mul(p2, a);
    }
  }
  public int Div(int a, int b)
  {
    return Mul(a, Pow(b, M - 2));
  }
  public int Ncr(int n, int r)
  {
    if (n < r) { return 0; }
    if (n == r) { return 1; }
    int res = Fac(n);
    res = Div(res, Fac(r));
    res = Div(res, Fac(n - r));
    return res;
  }
}

class Scanner
{
  string[] s;
  int i;

  char[] cs = new char[] { ' ' };

  public Scanner()
  {
    s = new string[0];
    i = 0;
  }

  public string next()
  {
    if (i < s.Length) return s[i++];
    string st = Console.ReadLine();
    while (st == "") st = Console.ReadLine();
    s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
    if (s.Length == 0) return next();
    i = 0;
    return s[i++];
  }

  public int nextInt()
  {
    return int.Parse(next());
  }
  public int[] ArrayInt(int N, int add = 0)
  {
    int[] Array = new int[N];
    for (int i = 0; i < N; i++)
    {
      Array[i] = nextInt() + add;
    }
    return Array;
  }

  public long nextLong()
  {
    return long.Parse(next());
  }

  public long[] ArrayLong(int N, long add = 0)
  {
    long[] Array = new long[N];
    for (int i = 0; i < N; i++)
    {
      Array[i] = nextLong() + add;
    }
    return Array;
  }

  public double nextDouble()
  {
    return double.Parse(next());
  }


  public double[] ArrayDouble(int N, double add = 0)
  {
    double[] Array = new double[N];
    for (int i = 0; i < N; i++)
    {
      Array[i] = nextDouble() + add;
    }
    return Array;
  }
}