function Main(input) {
    input = input.split(" ");
    nums = input.map(input=>parseInt(input,10))
    
    //var AB = parseInt(input[0], 10);
    //var BC = parseInt(input[1], 10);
    //var CA = parseInt(input[2], 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"));