"use strict";
process.stdin.resume();
process.stdin.setEncoding("utf8");


var reader = require("readline").createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on("line", line => {
  let num = line.split(/\s/).map(i => parseInt(i));

  console.log(num[0] * num[1] / 2);
});
