width, size_long = map(int, input().split())
if (width == 1 and size_long == 1):
    print(0)
else:
    print((width * size_long) // 2)
4e9206fa45c82cfe82a9b42e7cf2e928b86819d6
<mask token>

<mask token>
if width == 1 and size_long == 1:
    print(0)
else:
    print(width * size_long // 2)

width, size_long = map(int, input().split())
if width == 1 and size_long == 1:
    print(0)
else:
    print(width * size_long // 2)

width, size_long = map(int, input().split())
if (width == 1 and size_long == 1):
    print(0)
else:
    print((width * size_long) // 2)