#!usr/bin/env python3
from collections import defaultdict,deque
from heapq import heappush, heappop
import sys
import math
import bisect
import random
def LI(): return [int(x) for x in sys.stdin.readline().split()]
def I(): return int(sys.stdin.readline())
def LS():return [list(x) for x in sys.stdin.readline().split()]
def S(): return list(sys.stdin.readline())[:-1]
def IR(n):
    return [I() for i in range(n)]
def LIR(n):
    return [LI() for i in range(n)]
def SR(n):
    return [S() for i in range(n)]
def LSR(n):
    return [LS() for i in range(n)]

sys.setrecursionlimit(1000000)
mod = 1000000007

#A
def A():
    try:
        n = I()
        a = IR(n)
        l = []
        for i in range(n):
            for j in range(i):
                l.append(a[i]+a[j])
        l = list(set(l))
        l.sort()
        l = l[::-1]
        print(l[1])
    except:
        print('"assert"')
    return

#B
def B():
    n = I()

    return

#C
def C():
    h,w = LI()
    ans = max(0,h-w)+w*(w+1)//2
    print(ans)
    return

#D
def D():
    n = I()

    return

#E
def E():
    n = I()

    return

#F
def F():
    n = I()

    return

#Solve
if __name__ == "__main__":
    C()