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 LuaJIT (2.0.4) (judge verdict: Accepted)

local _i={}
for i=1,1 do
    local a=io.read()
    _i[i]={}
    for s in string.gmatch(a,"%S+")do
        _i[i][#_i[i]+1]=s
    end
end

local a={tonumber(_i[1][1]),tonumber(_i[1][2]),tonumber(_i[1][3])}
table.sort(a)
print(a[1]*a[2]*.5)