1113A - Sasha and His Trip - CodeForces Solution

n, v = (int(i) for i in input().split())
tank, res = 0, 0
for i in range(1, n + 1):
    n -= 1
    res += max(0, i * (min(v, n) - tank))
    tank = min(v, n) - 1
print(res)