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-----
3 4 5

-----Sample Output-----
6

This triangle has an area of 6.

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