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)
1490C - Sum of Cubes | 868A - Bark to Unlock |
873B - Balanced Substring | 1401D - Maximum Distributed Tree |
1716C - Robot in a Hallway | 1688B - Patchouli's Magical Talisman |
99A - Help Far Away Kingdom | 622B - The Time |
1688C - Manipulating History | 1169D - Good Triple |
1675B - Make It Increasing | 588A - Duff and Meat |
1541B - Pleasant Pairs | 1626B - Minor Reduction |
1680A - Minimums and Maximums | 1713A - Traveling Salesman Problem |
1713B - Optimal Reduction | 1710A - Color the Picture |
1686B - Odd Subarrays | 251A - Points on Line |
427C - Checkposts | 1159A - A pile of stones |
508A - Pasha and Pixels | 912A - Tricky Alchemy |
1249A - Yet Another Dividing into Teams | 1713C - Build Permutation |
1699A - The Third Three Number Problem | 1617B - GCD Problem |
841A - Generous Kefa | 1690B - Array Decrements |