N, K = [int(i) for i in input().split()]

def sum(a, b):
    return (a + b) * (b - a + 1) // 2

result = sum(1, N) - sum(1, N - K) + sum(N - K + 1, N)
if result % 2 == 0:
    print(result // 2 + 1)
else:
    print(result // 2)