L, K = map(int, input().split())
L_half = int(L/2)

if(L < 2*K):
    y = 0
elif(K == 1):
    if(L%2 == 0):
        y = L_half - 1
    else:
        y = L_half 
elif(L_half%K != 0):
    y = int(L_half/K) * K
else:
    y = int(L_half/K - 1) * K
    
print(y)