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)

n=int(input())
h=list(map(int, input().split()))
sum=0
for i in range(max(h)):
    if 0 not in h:
        h=[i-1 if i>0 else 0 for i in h]
        sum+=1
        # print(h,1)
    else:
        count0=[i for i, j in enumerate(h) if j==0]
        a=count0.copy() #copy()をつけないとaを変えるとcount0も変わる
        b=count0.copy()
        a.pop(0)
        b.pop(-1)
        c=[x-y for(x,y) in zip(a,b)]#a-bが要素のリストを作る
        sum = sum + len(c)-c.count(1)
        # print(count0)
        if count0[0]>0 and count0[-1]<n-1:
            sum +=2
            # print(1)
        elif count0[0]>0 or count0[-1]<n-1:
            sum +=1
            # print(2)
        else:
            sum +=0
        h=[i-1 if i>0 else 0 for i in h]
        # print(h,2)
print(sum)