r, c = map(int, input().split())

ans = 0

if r == c:
     if r % 2 == 0 and c % 2 == 0:
        ans = r * c // 4 - 1
     elif r % 2 != 0 and c % 2 != 0:
        ans = r * c // 4
else:
    if r % 2 == 0 or c % 2 == 0:
        ans = r * c // 2 - 1
    elif r % 2 != 0 and c % 2 != 0:
        ans = r * c // 2
print(ans)