#int(input())
#map(int, input().split())
#list(map(int, input().split()))

R, C = map(int, input().split())

a, b = divmod(R, 2)
c, d = divmod(C, 2)

if R == C:
    if b == 0:
        ans = a ** 2 - 1
    else:
        ans = a * (a + 1)
else:
    ans = R*C//2 + R*C%2 - 1

print(ans)