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: Accepted)

from statistics import mean, median,variance,stdev
import sys
import math
import fractions

def j(q):
    if q==1: print("YES")
    else:print("NO")
    exit(0)


def ct(x,y):
    if (x>y):print("")
    elif (x<y): print("")
    else: print("")

def ip():
    return int(input())

#x = ip()                              #入力整数1つ
#x,y= (int(i) for i in input().split())      #入力整数横2つ
x,y,z = (int(i) for i in input().split())    #入力整数横3つ
#n,m,x,y= (int(i) for i in input().split())  #入力整数横4つ
#x = [int(i) for i in input().split()]        #入力整数配列
#a = input()                                  #入力文字列
#a = input().split()                          #入力文字配列

#jの変数はしようできないので注意


print(x*y//2)