N,M=map(int,input().split())
if N%2==0 and M%2==0:
    print(0)
    exit()
elif N%2==0:
    print(M//4)
    exit()
if M%2==0:
    print(N//4)
else:
    if N%4==0:
        print(M)
    elif M%4==0:
        print(N)
    else:
        print(N//4+M//4)