You are given a rectangular board of *M* × *N* squares. Also you are given an unlimited number of standard domino pieces of 2 × 1 squares. You are allowed to rotate the pieces. You are asked to place as many dominoes as possible on the board so as to meet the following conditions:

1. Each domino completely covers two squares.

2. No two dominoes overlap.

3. Each domino lies entirely inside the board. It is allowed to touch the edges of the board.

Find the maximum number of dominoes, which can be placed under these restrictions.
In a single line you are given two integers *M* and *N* — board sizes in squares (1 ≤ *M* ≤ *N* ≤ 16).
def function(m, n):
    
    smallest = min(m,n)
    largest = max(m,n)
    sum =0
    extra=0
    
    
    if m==0 or n==0:
        return sum, extra
    
    
    if smallest == 1:
        if max(m,n) % 2 == 0:
            sum = sum + max(m,n)/2
            
            return sum, extra
        else:
            sum = sum + (max(m,n)-1)/2
            extra+=1
            
            return sum, extra
            
    if smallest % 2 == 0:
        temp_square = (smallest * smallest / 2)
        
        if m == largest:
            m -= smallest
        else:
            n -=smallest

        temp_sum , temp_extra = function(smallest, largest-smallest)
        sum  = temp_sum + temp_square
        extra += temp_extra
        
        print(sum)
        
    else:
        temp_squeare = ((smallest-1) * (smallest-1) / 2) + (smallest-1)
        square_extra = 1
        
        if m == largest:
            m -= smallest
        else:
            n -=smallest

        
        temp_sum , temp_extra = function(smallest, largest-smallest)
        sum  = temp_squeare + temp_sum
        extra += temp_extra
        extra += square_extra
        
    if m == n:
        return sum, extra
    temp_sum , temp_extra = function(smallest, max(m,n)-smallest)
    sum  = sum + temp_sum
    extra += temp_extra
        
    return sum, extra
n, m = input().split(' ')
n = int(n)
m = int(m)

temp_sum , temp_extra = function(m, n)
if temp_extra == 2:
    print(int(temp_sum)+1)
else:
    print(int(temp_sum))