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 <string.h>
#include <math.h>
 
int main()
{
  int  a, b, c, d, X, Y;
//  double  c;
//  long long  a, b, c, d, X, Y;
  int D, G;
  //char s[101][101]={};
  char sv[101]={};
  char ss[101]={};
//  char s_temp[101]={};
  char t[12]={};
//  char s[10000000000000000001]={};
  char s[2][100001]={};
  int len=0;
//  int N, M;
//  int A[1000000001]={};
  long long  A[101]={};
//  long int A[1001]={};
  long long  B[101]={};
  long long  C[101]={};
  int i,j,k;
  int flag=0;
  int tmp=0;
  int min=1000;
  int cnt=0;
  double ans=1000;
  int max=0;

  int  Cen_X=0;
  int  Cen_Y=0;
  long long  H=0;
  
  //  int ans=0;
  int str_len=0;
//  char alp[26]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
  int ans_o[1001]={};
  double N[1001]={};
  int ans_e[100001]={};
//  a=0;
//  scanf("%lld %lld", &a, &b);
  scanf("%d %d %d", &a, &b, &c);
//  cnt = strlen(t);
  //printf("%d\n",cnt);
  
//  for(i=0;i<cnt;i++){
//    a=a+(t[cnt-i-1]-'0') * ()(pow(10, i));
//  printf("%d\n",min);
//  }
      
//  if(flag==0)   printf("TLE\n");
  printf("%d\n", (a*b)/2);
  
return 0;
}