# k = int(input())
# a = input()
# b = input()
# set_a = set()
# for i in range(0, len(a)-k+1):
# set_a.add(a[i:i+k])
# res = 0
# for i in range(0, len(b)-k+1):
# if b[i:i+k] in set_a:
# res += 1
# print(res)
"""
2
abab
ababab
"""
"""
7 14
"""
from math import sqrt
nums = [int(i) for i in input().split()]
x, y = nums[0], nums[1]
# n = 1
# while (n+1)*n/2 != sum(nums):
# n += 1
n = int((sqrt(8*sum(nums)+1)-1)/2)
permu = [i+1 for i in range(n)]
res = []
def exist(permu, x, path):
if not permu and x != 0:
return (False, -1)
if x == 0 or x in permu:
res.append(path+1)
return (True, path + 1)
if exist(permu[1:], x-permu[0], path+1)[0]:
res.append(exist(permu[1:], x-permu[0], path+1)[1])
return (True, exist(permu[1:], x-permu[0], path+1)[1])
if exist(permu[1:], x, path+1)[0]:
res.append(exist(permu[1:], x, path+1)[1])
return (True, exist(permu[1:], x, path+1)[1])
exist(permu, x, 0)
print(res)
print(min(res) if res else -1)
"""
2 3 3
"""
# res = 0
# def tri(a, b, c):
# if a + b > c and a + c > b and b + c > a:
# return True
# return False
#
# nums = [int(i) for i in input().split()]
# res = 0
# for i in range(1, nums[0]+1):
# for j in range(i+1, nums[1]+1):
# for k in range(j, nums[2]+1):
# if tri(i, j, k):
# res += 3
#
# print(res)
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
# from math import sqrt
# nums = [int(i) for i in input().split()]
# x, y = nums[0], nums[1]
#
# n = 1
# while (n+1)*n/2 != sum(nums):
# n += 1
#
# n = int((sqrt(8*sum(nums)+1)-1)/2)
#
# permu = [i+1 for i in range(n)]
# res = 0
# while x > 0:
# x -= permu[-1]
# permu = permu[:-1]
# res += 1
# print(res)
