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: Runtime Error)

#include <stdio.h>

int main(int argc, char *argv[])
{
  int s;
  scanf("%d", s);

  int a[1000000];
  int i = 1;
  a[0] = s;

  while(i < 1000000){
    if(a[i - 1] % 2 == 0){
      a[i] = a[i - 1] / 2;
    }
    else{
      a[i] = 3 * a[i - 1] + 1;
    }
    i++;
  }

  int j = 2;

  while(j != -1){
    for(int k = j - 1; k >= 0; k--){
      if(a[j] == a[k]){
	printf("%d\n", j + 1);
	j = -1;
	break;
      }
      else if(k == 0){
	j++;
      }
    }
  }

  return 0;
}