function Main(input) {
    input = input.split(" ");
    nums = input.map(input=>parseInt(input,10))

  　var AB = nums[0];
    var BC = nums[1];
    var CA = nums[2];
    var S = AB * BC/2;
    console.log(S);
}  

Main(require("fs").readFileSync("/dev/stdin", "utf8"));