def compute_Xn(x, n):
    if n == 0:
        return x
    k = int(( ( (8 * x + 1)**0.5 - 1 ) ) // 2)
    while k * (k + 1) // 2 >= x:
        k -= 1
    m = k + 1
    if n <= m:
        return x - n * (n + 1) // 2
    else:
        t = n - m
        xm = x - m * (m + 1) // 2
        return xm - (t // 2) + (t % 2) * (m + t)

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