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++14 (GCC 5.4.1) (judge verdict: Accepted)

#pragma region RegionDefs
#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#define rep(i,n) for(int i=0,i##_len=(n);i<i##_len;++i)
#define reps(i,j,n) for(int i=(j),i##_len=(n);i<i##_len;++i)
#define repr(i,n) for(int i=(n);i>=0;--i)
#define all(x) begin(x),end(x)
using namespace std;
const int INF = 1e9;
template<class T=int> using P  = pair<T, T>;
template<class T=int> using V  = vector<T>;
template<class T=int> using PQ = priority_queue<T>;
template<class T=int> using PQG = priority_queue<T, V<T>, greater<T>>;
typedef long long ll;
const ll MOD = 1000000007LL;
void in() {}
template<class Head, class... Tail>
void in(Head&& head, Tail&&... tail)
{
    cin >> head;
    in(move(tail)...);
}
#define IN(...) ll __VA_ARGS__; in(__VA_ARGS__)
//時間が厳しいとき使う
#define INI(...) int __VA_ARGS__; in(__VA_ARGS__)
#define INS(T, ...) T __VA_ARGS__; in(__VA_ARGS__)
#define VIN(T, v, n) V<T> v(n); for(auto& _elem:v)cin>>_elem
template<class T>bool chmax(T &a, const T &b) {if (a<b) {a=b; return 1;} return 0;}
template<class T>bool chmin(T &a, const T &b) {if (a>b) {a=b; return 1;} return 0;}
#define y0 y__0
#define y1 y__1
#define j0 j__0
#define j1 j__1
#define YN(e) cout << ((e) ? "YES" : "NO") << endl
#define yn(e) cout << ((e) ? "Yes" : "No") << endl
#pragma endregion RegionDefs

void solve()
{
    IN(h, w);
    if (h == 1 || w == 1)
    {
        cout << 1 << endl;
        return;
    }
    cout << (h * w + 1) / 2 << endl;
}

int main()
{
    cin.tie(0); cout.tie(0);
    ios::sync_with_stdio(false);
    cout << setprecision(numeric_limits<double>::max_digits10);

    solve();

    return 0;
}