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 Python (3.4.3) (judge verdict: Runtime Error)

a = []
i = 0

s = int(input())

a.append(s)

while True:
    if(a[i] % 2 == 0):
        if(a[i] / 2 != 0):
        	ans = int(a[i] / 2)
        else:
            ans = 0
        if ans in a:
            print(i + 2)
            break
       	a.append(ans)
    else:
        anss = int(a[i] * 3 + 1)
        if anss in a:
            print(i + 2)
            break
        a.append(anss)

    i += 1