[print(n.prod([int(i)for i in input().split()][:2])//2)for n in[__import__("numpy")]]

[print(a*b//2)for a,b in[[int(i) for i in input().split()][:2]]]

[
    print(n.prod([int(i) for i in input().split()][:2]) // 2)
    for n in [__import__("numpy")]
]

[print(a * b // 2) for a, b in [[int(i) for i in input().split()][:2]]]

-[ -    print(n.prod([int(i) for i in input().split()][:2]) // 2) -    for n in [__import__("numpy")] -] +[print(a * b // 2) for a, b in [[int(i) for i in input().split()][:2]]]