import core.thread;
import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.range, std.regex;

//	Input
class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { tokens = readln.split; if (stdin.eof) throw new EOFException; } auto token = tokens[0]; tokens.popFront; return token; }
int readInt() { return to!int(readToken); }
long readLong() { return to!long(readToken); }
real readReal() { return to!real(readToken); }

//	chmin/chmax
void chmin(T)(ref T t, in T f) { if (t > f) t = f; }
void chmax(T)(ref T t, in T f) { if (t < f) t = f; }

//	Pair
struct Pair(S, T) {
	S x; T y;
	int opCmp(    const Pair p) const { return (x < p.x) ? -1 : (x > p.x) ? +1 : (y < p.y) ? -1 : (y > p.y) ? +1 : 0; }
	int opCmp(ref const Pair p) const { return (x < p.x) ? -1 : (x > p.x) ? +1 : (y < p.y) ? -1 : (y > p.y) ? +1 : 0; }
	string toString() const { return "(" ~ to!string(x) ~ ", " ~ to!string(y) ~ ")"; }
}
auto pair(S, T)(inout(S) x, inout(T) y) { return Pair!(S, T)(x, y); }

//	Array
int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = as.length; for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) { return (a <  val); }); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) { return (a <= val); }); }
T[] unique(T)(in T[] as) { T[] bs; foreach (a; as) if (bs.empty || bs[$ - 1] != a) bs ~= a; return bs; }


long solveBrute(long X, long Y) {
	long ret;
	foreach (x; 0 .. X) foreach (y; 0 .. Y) {
		bool ok = true;
		foreach (xx; 0 .. X) foreach (yy; 0 .. Y) if (pair(xx, yy) < pair(x, y)) {
			if (X == Y) {
				if (pair(x, y) == pair(Y - 1 - yy, xx)) ok = false;
				if (pair(x, y) == pair(X - 1 - xx, Y - 1 - yy)) ok = false;
				if (pair(x, y) == pair(yy, X - 1 - xx)) ok = false;
			} else {
				if (pair(x, y) == pair(X - 1 - xx, Y - 1 - yy)) ok = false;
			}
		}
		if (ok) {
			++ret;
		}
	}
	--ret;
	return ret;
}

long solve(long X, long Y) {
	long ret;
	if (X == Y) {
		ret = (X * Y + 3) / 4;
	} else {
		ret = (X * Y + 1) / 2;
	}
	--ret;
	return ret;
}

unittest {
	foreach (X; 1 .. 10) foreach (Y; 1 .. 10) {
		const expected = solveBrute(X, Y);
		const actual = solve(X, Y);
		if (expected != actual) {
			writeln("X = ", X, ", Y = ", Y, " : expected ", expected, ", actual ", actual);
			return;
		}
	}
}

void main(string[] args) {
	try {
	for (; ; ) {
		const X = readLong;
		const Y = readLong;
		
		const res = solve(X, Y);
		writeln(res);
	}
	} catch (EOFException) {}
}