p03145 AtCoder Beginner Contest 116 - Right Triangle
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.

Examples

Input

3 4 5


Output

6


Input

5 12 13


Output

30


Input

45 28 53


Output

630
```python
a,b,c=map(int,input().split())
print(str(int(0.5*a*b)))
```