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.

Examples

Input

3 4 5


Output

6


Input

5 12 13


Output

30


Input

45 28 53


Output

630
import sys
from collections import deque
import copy
import math
def get_read_func(fileobject):
    if fileobject == None :
        return raw_input
    else:
        return fileobject.readline

def main():
    if len(sys.argv) > 1:
        f = open(sys.argv[1])
    else:
        f = None
    read_func = get_read_func(f);
    input_raw = read_func().strip().split()
    [A, B, C] = [long(input_raw[0]), long(input_raw[1]), long(input_raw[2])]
    print A * B /2

if __name__ == '__main__':
    main()
