function Main(input) {
    input = input.split(" ");

    var AB = parseInt(input[0], 10);
    var BC = parseInt(input[1], 10);
    var CA = parseInt(input[2], 10);
   
    var S = AB * BC/2;
    console.log(S);
}  

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