var area = (input) => {
  var lengthList = input.split(' ');
  var varticalLength = Number.parseInt(lengthList[0]);
  var horizontalLength = Number.parseInt(lengthList[1]);
  
  console.log(`${varticalLength * horizontalLength / 2}`);
}

process.stdin.resume();
process.stdin.setEncoding('utf8');
var inputString = '';

process.stdin.on('data', function(chunk) {
  inputString += chunk;
});

process.stdin.on('end', function() {
  area(inputString);
});