function Main(input){
	var input= input.split(' '),
	H = input[0],
	W = input[1];
	if(H == 1 || W == 1){
		console.log(0);
	}else{
		console.log(H*W%2 == 0 ? H*W/2 : (H*W+1)/2);
	}
	
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));