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++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n+1); ++ (i))
#define REPR(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define FORR(i, m, n) for(int i=(m)-1; i>=(n); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(),(c).end())
#define SORTR(c) sort((c).begin(),(c).end(),greater<int>());
#define BIT(n) (1LL<<(n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x,y) __gcd(x,y)
#define LCM(x,y) x/__gcd(x,y)*y
#define PB push_back
#define MP make_pair
#define endl "\n"
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
using namespace std;
using Graph = vector<vector<int> >;
using WeightedGraph = vector<vector<pair<int, int> > >;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int> > VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
const int INF = 2000000000;
const long double PI = acosl(-1);
const int MOD = 1000000007;
void print() {cout << endl;}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head; if (sizeof...(tail) != 0) cout << " ";print(forward<Tail>(tail)...);}
template <class T> void print(vector<T> &vec) {for (auto& a : vec) {cout << a; if (&a != &vec.back()) cout << " ";} cout << endl;}
template <class T> void print(vector<vector<T>> &df) { for (auto& vec : df) { print(vec);}}


signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cout << fixed << setprecision(10);
    int ans = 0;
    
    int H, W;
    cin >> H >> W;

    if (H == 1 || W == 1) ans = 1;
    else {
        ans = (H*W+1)/2;
    }
    cout << ans << endl;
    
    
    
    return 0;
}

/*
cd ~/Desktop/Programming/AtCoder/contest/panasonic2020/panasonic2020_b && sh shellscripts/test_cpp.sh
cd ~/Desktop/Programming/AtCoder/contest/panasonic2020/panasonic2020_b && sh shellscripts/etest_cpp.sh
cd ~/Desktop/Programming/AtCoder/contest/panasonic2020/panasonic2020_b && sh shellscripts/submit_cpp.sh
*/