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

if R!=C:
    if R%2==1 and C%2==1:
        print((R*C)//2)
    else:
        print((R*C)//2-1)
else:
    if R%2==1 and C%2==1:
        print((R*C)//4)
    else:
        print((R*C)//4-1)