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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include<stdio.h>
int main(){
  int s,a[1000000];
  scanf("%d",&s);
  a[1] = s;
  for(int i = 2;i < 1000000;i++){
    a[i] = f(a[i - 1]);
    for(int j = 1;j < i;j++){
      if(a[i] == a[j]){
        printf("%d",i);
        return 0;
      }
    }
  }
}

int f(int n){
  if(n % 2 == 0){
    return n / 2;
  }else{
    return 3 * n + 1;
  }
}