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

inputFormat

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|

outputFormat

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

样例

3 4 5

6