L, K = map(int, input().split())
y = 0

if(K == 1):
    if(L%2 == 0):
        y = int(L/2) -1
    else:
        y = int(L/2)
    L = 0
    
while(L > 2*K):
    y += K
    L -= 2*K
    
print(y)