N, M = map(int, input().split())

if N == 1 or M == 1:
    K = (N * M) // 4
    print(K)
else:
    r = (N % 2) * (M % 2)
    x = (N * M - r) // 4
    K = (N * M) // 4
    I_min = max(0, K - x)
    print(I_min)