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

ng, ok = -1, 2 * 10**9
while ok - ng > 1:
    mid = (ok + ng) // 2
    if mid * (mid + 1) // 2 > x:
        ok = mid
    else:
        ng = mid

k = min(n, ok)
x -= k * (k + 1) // 2
rem = (n - k) // 2
x -= rem
if k + rem * 2 != n:
    x += n
print(x)