lis = list(map(int,input().split()))
if(lis[0]%lis[1] == 0 and lis[0]/lis[1] == 2):
    print(0)
else:
    if(lis[0]%lis[1] == 0):
        print(int((lis[0]/2)//lis[1]*lis[1]-lis[1]))
    else:
        print(int((lis[0]/2)//lis[1]*lis[1]))