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: Wrong Answer)

#include <stdio.h>

int f(int a){
  return (a % 2 == 0 ? (a / 2) : (3 * a + 1));
}
  
int main(){
  int s; scanf("%d", &s);
  int flag = 0;
  int mem[10000] = {};
  int count = 1;
  mem[s] = 1;
  int tmp = s;
  
  do{
    count++;
    if(mem[f(tmp)] == 1) flag = 1;
    mem[tmp] = 1;
    tmp = f(tmp);
  }while (flag == 0);

  printf("%d", count);

  return 0;
}