x,n = map(int,input().split())
c = 1
if x < 0:
    x *= -1
    c = -1
l,r = 0,10**18
for i in range(60):
    mid = (l+r)//2
    z = (1 + mid) * mid // 2
    if z > x:
        r = mid
    else:
        l = mid

if x == (l + 1) * l // 2:
    l -= 1
if l >= n:
    ans = x - ((1 + n) * n // 2)
    print(ans * c)
    exit()
x -= (l + 1) * l // 2
y = l + 1
ans = x + (n - y + 1) // 2
if (n - y + 1) % 2 == 1:
    ans -= n
print(ans * c)