x, n = map(int, input().split())

minus = (x <= 0)
if minus:
    x *= -1

ng = 0
ok = 2000000001
while ok - ng > 1:
    mid = (ok + ng) // 2
    if mid * (mid + 1) // 2 <= x:
        ng = mid
    else:
        ok = mid

after_n = x - ok * (ok + 1) // 2
after_p = after_n + ok + 1

ans = 0

if n <= ok:
    ans = x - n * (n + 1) // 2
elif n % 2 == ok % 2:
    ans = after_n - (n - ok) // 2
else:
    ans = after_p + (n - (ok + 1)) // 2
if minus:
    ans *= -1

print(ans)