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 C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define mod 10e9+7
#define ll long long int

long long int xor(long long int x, long long int y){
    long long int x2len = floor(log2(x)) + 1;
    long long int y2len = floor(log2(y)) + 1;
    long long int len;
    if(x2len > y2len) len = x2len;
    else len = y2len;
    long long int z = 0;
    for(int i = 0;i < len;i++){
        z += pow(2, i) * ((x%(long long int)pow(2,i+1) >= pow(2, i))!=(y%(long long int)pow(2,i+1) >= pow(2, i)));
    }
    return z;
}

int main(void){
    int a, b, c;
    scanf("%d %d %d", &a, &b, &c);
    if(a < c || b < c)printf("%d\n", a*b/2);
    else if(b < a || c < a)printf("%d\n", b*c/2);
    else printf("%d\n", c*a/2);
    return 0;
}