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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

main = do
  [a,b,c] <- map read . words <$> getLine :: IO [Int]
  if a ^ 2 + b ^ 2 == c ^ 2
    then print $ a * b `div` 2
    else if a ^ 2 + c ^ 2 == b ^ 2
           then print $ a * c `div` 2
           else print $ b * c `div` 2