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 Perl (v5.18.2) (judge verdict: Accepted)

    use warnings;
    use Data::Dumper;
    
    sub GetInputData{
    
        my @array_data;
    
        my $line = <STDIN>;
    
        chomp($line);
    
        while($line ne ""){
    
           push(@array_data,$line);
    
           $line = <STDIN>;
    
           chomp($line);
    
    }
    
        return @array_data;
    
    }
    
    sub GetSpaceInputData{
    
        my @array_temp=GetInputData();
    
        my @array_buf;
    
        for(@array_temp){
    
            #戻りは配列スペース区切りを分解
            # myを先頭につける必要あり
            my @array_work=split(" ",$_);

            # リファレンスを配列に代入
            push(@array_buf,\@array_work);
        }
    
        return @array_buf;
    
    }
    
    my @arr_result = GetSpaceInputData();

    my $ab = $arr_result[0][0];
    my $bc = $arr_result[0][1];

    print $ab*$bc / 2;