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

if R == C == 1:
    exit(print(0))

if R == C:
    if R%2 == 0:
        cnt = (R//2) ** 2
    else:
        cnt = (R//2) ** 2
        cnt += (R + 2 - 1)//2    
else:
    if R%2 == 1 and C%2 == 1:
        cnt = 1 + R*C//2
    else:
        cnt = R*C//2

print(cnt - 1)