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 Kotlin (1.0.0) (judge verdict: Wrong Answer)

import java.io.PrintWriter
import java.util.*
import java.math.*
import java.lang.*
import kotlin.comparisons.*

val pw = PrintWriter(System.out)
val MOD: Long = (1e+9 + 7).toLong()

fun main(args: Array<String>) {
    val (h, w) = readListOfLong()

    val hp = if(h%2 == 1L) 1L else 0L
    val wp = if(w%2==1L) 1L else 0L
    val ans = (h/2 + hp) * (w/2 + wp) + (h/2) * (w/2) 
    println(ans)
	pw.flush()
}

/****** Declared Functions and Data Structures ******/
// IO
fun read() = readLine()!!
fun readInt() = read().toInt()
fun readLong() = read().toLong()
fun readDouble() = read().toDouble()
fun readListOfString() = read().split(" ")
fun readListOfInt() = readListOfString().map { it.toInt() }
fun readListOfLong() = readListOfString().map { it.toLong() }
fun readListOfDouble() = readListOfString().map { it.toDouble() }

fun Double.format(digits: Int) = String.format("%.${digits}f", this)
fun Float.format(digits: Int) = String.format("%.${digits}f", this)

fun print(value: Any) { pw.print(value) }
fun println(value : Any) { pw.println(value) }

// Extensions
fun<T> List<T>.toBuckets(): Map<T, Int>
	= this.groupBy { it }.mapValues { it.value.size }

fun<T: Comparable<T>> List<T>.upperBound(element: T, fromIndex: Int=0, toIndex: Int=this.size): Int {
	var l = fromIndex
	var r = toIndex
	while(l < r) {
		val mid = (l+r) / 2
		if(element >= this[mid]) { 
			l = mid + 1
		} else {
			r = mid
		}
	}
	return l
}

fun<T: Comparable<T>> List<T>.lowerBound(element: T, fromIndex: Int=0, toIndex: Int=this.size): Int {
	var l = fromIndex
	var r = toIndex
	while(l < r) {
		val mid = (l+r) / 2
		if(element <= this[mid]) { 
			r = mid
		} else {
			l = mid + 1
		}
	}
	return r 
}

fun <K, V> Map<K, V>.toMutableMap(): MutableMap<K, V> = HashMap(this)

// Util Functions
fun Long.makeDivisors(): List<Long> {
	val divisors = mutableListOf<Long>()
	for(i in 1L .. Math.sqrt(this.toDouble()).toLong()+1L) {
		if(this % i == 0L) {
			divisors.add(i)
			if(i != this/i) {
				divisors.add(this/i)
			}
		}
	}
	return divisors.distinct().sorted()
}

fun Long.isPrime(): Boolean {
	if(this==1L) return false
	for(i in 2L .. Math.sqrt(this.toDouble()).toLong() + 1L) {
		if(this % i == 0L && this != i) {
			return false
		}
	}
	return true
}


fun<T> permutations(src: List<T>): List<List<T>> {
	if(src.size == 1) return listOf(src)
	val perms = mutableListOf<List<T>>()
	val insertElement = src[0]
	permutations(src.drop(1)).forEach { perm ->
		for(i in 0..perm.size) {
			val newPerm = perm.toMutableList()
			newPerm.add(i, insertElement)
			perms.add(newPerm.toList())
		}
	}
	return perms
}

// return nCr, if you want nCr, please access v[n][r]
fun combinations(n: Long): List<List<Long>> {
	val v = (0..n).map { (0..n).map{0L}.toMutableList() }.toMutableList()
	for(i in 0 until v.size) {
		v[i][0] = 1L
		v[i][i] = 1L
	}
	for(i in 0 until v.size) {
		for(j in 1 until i) {
			v[i][j] = (v[i-1][j-1] + v[i-1][j]) % MOD
		}
	}
	return v.map { it.toList() }.toList()
}

fun repeatedModPow(a: Long, n: Long, mod: Long): Long = 
	when {
		n == 1L -> a % mod
		n % 2L == 1L -> (a * repeatedModPow(a, n-1L, mod)) % mod
		else -> {
			val tmp = repeatedModPow(a, n / 2L, mod)
			(tmp * tmp) % mod
		}
	}

// return nCr % MOD
fun modnCkWithoutArray(n: Long, k: Long, mod: Long): Long {
	var ret = 1L
	val min = Math.min(k, n-k)
	// まず分母の計算
	for(i in n downTo (n - min + 1)) {
		ret = ret * i % mod
	}
	for(i in min downTo 1) {
		ret = ret * repeatedModPow(i, (mod - 2L), mod) % mod
	}
	return ret
}

// return nCr % MOD
fun modnCk(n: Int, k: Int, mod: Long): Long {
	val factorials = Array(n+1) { 0L }.apply {
		this[0] = 1L
		if(this.size > 1) this[1] = 1L
	}
	val inverses = Array(n+1) { 0L }.apply {
		this[0] = 1L
		if(this.size > 1) this[1] = 1L
	}
	val factorialInverses = Array(n+1) { 0L }.apply {
		this[0] = 1L
		if(this.size > 1) this[1] = 1L
	}
	for(i in 2 .. n) {
		factorials[i] = (factorials[i-1] * i.toLong() % mod)
		inverses[i] = (mod - inverses[mod.toInt() % i] * (mod / i.toLong()) % mod)
		factorialInverses[i] = (factorialInverses[i-1] * inverses[i] % mod)
	}

	return factorials[n] % mod * (factorialInverses[k] * factorialInverses[n-k] % mod) % mod
}

// should be a >= b
fun gcd(a: Long, b: Long): Long = 
	if(b == 0L) a else gcd(b, a % b)

// shoud be a >= b
fun lcm(a: Long, b: Long): Long = 
	a / gcd(a, b) * b

fun sumDigits(num_: Long): Long {
	var num = num_
	var rtn: Long = 0
	while(num != 0.toLong()) {
		val tmp = num % 10
		rtn += tmp
		num /= 10
	}
	return rtn
}

fun Double.isDecimal(): Boolean = ((this - Math.floor(this)) != 0.0)

// Data Structures

class UnionFind(private val n: Int) {
	private val parent: Array<Int> = Array(n) { 0 }
	private val rank: Array<Int> = Array(n) { 0 }
	private val size: Array<Int> = Array(n) { 0 }

	init {
		for(i in 0 until n) {
			parent[i] = i
			rank[i] = 1
			size[i] = 1
		}
	}

	fun union(u: Int, v: Int) {
		var x = root(u)
		var y = root(v)
		if(x != y) {
			if(rank[y] < rank[x]) {
				val tmp = y 
				y = x
				x = tmp 
			}
			parent[x] = y
			rank[y] += rank[x]
			rank[x] = -1
			val totalSize = size[x] + size[y]
			size[x] = totalSize
			size[y] = totalSize
		}
	}

	fun root(u: Int): Int {
		var x = u
		while(parent[x] != x) {
			x = parent[x]
		}
		return x 
	}

	fun size(u: Int): Int = size[root(u)]
}

class Stack<T>(private var st: MutableList<T> = mutableListOf<T>()) {
	fun isEmpty() = st.isEmpty()
	fun top(): T = st.last()
	fun push(e: T) { st.add(e) }
	fun pop() { st = st.dropLast(1).toMutableList() }
}

class Queue<T>(private var que: MutableList<T> = mutableListOf<T>()) {
	fun isEmpty() = que.isEmpty()
	fun top(): T = que.first()
	fun push(e: T) { que.add(e) }
	fun pop() { que = que.drop(1).toMutableList() }
}