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 Fortran (GNU Fortran 9.2.1) (judge verdict: Wrong Answer)

program prob27
    implicit none
    integer :: a,b,c
    write(*,*) a,b,c
    if(a <= c .and. b <= c) then
        write(*,*) a * b / 2
    else if(a <= b .and. c <= b) then
        write(*,*) a*c/2
    else
        write(*,*) b*c/2
    end if

    stop
contains
end program prob27