INF = 10 ** 9
MOD = 10 ** 9 + 7
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)
from  copy import deepcopy
from bisect import bisect_left

def main():
    n,k = map(int,input().split())
    M = n * (n + 1)//2
    m = - M + (n - k) * (n - k + 1)
    print((M - m)//2 + 1) 
if __name__ == '__main__':
    main()