Problem:
Score : 100 points

Problem Statement

There is a right triangle ABC with ∠ABC=90°.

Given the lengths of the three sides, |AB|,|BC| and |CA|, find the area of the right triangle ABC.

It is guaranteed that the area of the triangle ABC is an integer.

Constraints

1 \leq |AB|,|BC|,|CA| \leq 100

All values in input are integers.

The area of the triangle ABC is an integer.

Input

Input is given from Standard Input in the following format:

|AB| |BC| |CA|

Output

Print the area of the triangle ABC.

Sample Input 1

3 4 5

Sample Output 1

6

This triangle has an area of 6.

Sample Input 2

5 12 13

Sample Output 2

30

This triangle has an area of 30.

Sample Input 3

45 28 53

Sample Output 3

630

This triangle has an area of 630.

Solution in JavaScript (node.js v5.12) (judge verdict: Wrong Answer)

var num = +(require('fs').readFileSync('/dev/stdin', 'utf8').trim());
var odds = (n) => { return n >> 1; };
var ends = (n) => { return 3 * n + 1; };
var r = [num], i;
if (num == 1) {
  console.log(num);
  return;
}
while (num > 4) {
  num = (num % 2 == 0 ? odds(num) : ends(num));
  i = r.lastIndexOf(num);
  // console.log(i);
  if (i > -1) {
    // console.log(r);
    console.log(r.length + 1);
    return;
  }
  r.push(num);
}
// console.log(r);
console.log(r.length + 3);