# def search(n,k):
# s = 0
# a=1
# f=1
# if n == k:
# return 1
# for i in range(k):
# if n ==1:
# break
# a = a * (n)
# n = n-1
# for i in range(k):
# f = f * k
# k = k-1
#
# s = a // f
# return s
# n, k = map(int, input().split(' '))
# print(search(n,k))
def case(n,k,m):
a=0
s=0
n = n-k
if m < 2 * k:
return 0
if k==1:
return m
if m == k:
return 0
for i in range(k):
a = a + (n)
n = n-1
s = a
return s
n, k =map(int, input().split(' '))
m=n
print(case(n, k, m))