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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
	"strings"
	"unsafe"
)

func dfs(x int, y int, used map[Pair]int, maxW int, maxH int) {

	// used[Pair{first: x, second: y}] = 1
	// xArr := []int{1, 1, -1, -1}
	// yArr := []int{1, -1, -1, 1}
	// for i := 0; i < 4; i++ {
	// 	nx := x + xArr[i]
	// 	ny := y + yArr[i]
	// 	if used[Pair{first: nx, second: ny}] == 1 {
	// 		continue
	// 	}
	// 	if nx > maxW || ny > maxH || nx < 0 || ny < 0 {
	// 		continue
	// 	}
	// 	dfs(nx, ny, used, maxH, maxW)
	// }
}

func main() {
	H, W := sc.nextInt(), sc.nextInt()
	if H == 1 || W == 1 {
		fmt.Println(1)
		return
	}
	if H*W%2 == 0 {
		fmt.Println(H * W / 2)
	} else {
		fmt.Println((H * W / 2) + 1)
	}
	// field := make([][]int, H)
	// for i := 0; i < H; i++ {
	// 	field[i] = make([]int, W)
	// }
	// mm := make(map[Pair]int)
	// dfs(0, 0, mm, W-1, H-1)
	// fmt.Println(len(mm))
	// fmt.Println((mm))
}

var (
	sc scanner
)

func init() {
	sc = scanner{
		buf: make([]string, 0, 0),
		cur: 0,
		r:   bufio.NewReader(os.Stdin),
	}
}

type scanner struct {
	buf []string
	cur int
	r   *bufio.Reader
}

func (s *scanner) readln() {
	rbuf := make([]byte, 0, 0)
	for {
		line, prefix, err := s.r.ReadLine()
		if err != nil {
			panic(err)
		}
		rbuf = append(rbuf, line...)
		if prefix == false {
			break
		}
	}
	s.cur = 0
	s.buf = strings.Split(*(*string)(unsafe.Pointer(&rbuf)), " ")
}
func (s *scanner) isFull() bool {
	if s.cur == len(s.buf) {
		return true
	}
	return false
}
func (s *scanner) resetCur() {
	s.cur = 0
}
func (s *scanner) next() string {
	if s.cur == 0 {
		s.readln()
	}
	res := s.buf[s.cur]
	s.cur++
	if s.isFull() {
		s.resetCur()
	}
	return res
}
func (s *scanner) nexts() []string {
	s.readln()
	s.resetCur()
	return s.buf
}
func (s *scanner) nextInt() int {
	if s.cur == 0 {
		s.readln()
	}
	res, _ := strconv.Atoi(s.buf[s.cur])
	s.cur++
	if s.isFull() {
		s.resetCur()
	}
	return res
}
func (s *scanner) nextInts() []int {
	s.readln()
	res := make([]int, len(s.buf))
	for i := range s.buf {
		res[i], _ = strconv.Atoi(s.buf[i])
	}
	s.resetCur()
	return res
}
func (s *scanner) nextFloat() float64 {
	if s.cur == 0 {
		s.readln()
	}
	res, _ := strconv.ParseFloat(s.buf[s.cur],
		64)
	s.cur++
	if s.isFull() {
		s.resetCur()
	}
	return res
}
func (s *scanner) nextFloats() []float64 {
	s.readln()
	res := make([]float64, len(s.buf))
	for i := range s.buf {
		res[i], _ = strconv.ParseFloat(s.buf[i],
			64)
	}
	s.resetCur()
	return res
}
func digits(x int) int {
	return len(strconv.Itoa(x))
}
func powInt(x, p int) (result int) {
	result = 1
	for i := 0; i < p; i++ {
		result *= x
	}
	return
}
func max(x, y int) int {
	return int(math.Max(float64(x), float64(y)))
}
func min(x, y int) int {
	return int(math.Min(float64(x), float64(y)))
}
func abs(x int) int {
	return int(math.Abs(float64(x)))
}
func varDump(value ...interface{}) {
	for _, v := range value {
		fmt.Fprintf(os.Stderr, "%#v\n", v)
	}
}
func yes() {
	fmt.Println("Yes")
}
func no() {
	fmt.Println("No")
}

// Pair a
type Pair struct {
	first  int
	second int
}

// Queue a
type Queue struct {
	v []Pair
}

// Push for queue
func (q *Queue) Push(v Pair) {
	q.v = append(q.v, v)
}

// Pop for queue
func (q *Queue) Pop() Pair {
	r := q.v[0]
	q.v = q.v[1:]
	return r
}

// Front for queue
func (q Queue) Front() Pair {
	return q.v[0]
}

// Empty return true if empty
func (q Queue) Empty() bool {
	return len(q.v) == 0
}