2021/06/06
import sys
A, B = map(int, sys.stdin.readline().split())
i = 1
size = 0
li = []
while (size < B):
tmp = i
while tmp:
li.append(i)
tmp-=1
i += 1
size = len(li)
ans = 0
for i in range(A - 1, B):
ans+=li[i]
print(ans)