For a positive integer n let's define a function f:

f(n) = - 1 + 2 - 3 + .. + ( - 1)nn

Your task is to calculate f(n) for a given integer n.

Input

The single line contains the positive integer n (1 ≤ n ≤ 1015).

Output

Print f(n) in a single line.

Examples

Input

4


Output

2


Input

5


Output

-3

Note

f(4) = - 1 + 2 - 3 + 4 = 2

f(5) = - 1 + 2 - 3 + 4 - 5 = - 3
# vowels = ["a", "o", "y", "e", "u", "i"]
# s = list(input().lower())
# ans = ""
# for i in vowels:
#     if i in s:
#         while i in s:
#             s.remove(i)
# for i in range(len(s)):
#     s[i] = "." + s[i]
# print(ans.join(s))


# n = int(input())
# ans = 0
# for i in range(n):
#
#     s = input()
#     if "-" in s:
#         ans -= 1
#     if "+" in s:
#         ans += 1
#
#
# print(ans)

# s1 = input().lower()
# s2 = input().lower()
# isequal = False
# for i in range(len(s1)):
#     if s1[i] > s2[i]:
#         print("1")
#         isequal = True
#         break
#     if s2[i] > s1[i]:
#         print("-1")
#         isequal = True
#         break
# if not isequal:
#     print("0")

# s = list(input())
# not_dangerous = False
# for i in range(len(s)):
#     if i != len(s) - 1:
#         j = 0
#         while i + j <= len(s) - 1 and s[i] == s[i + j]:
#             j += 1
#         if j >= 7:
#             print("YES")
#             not_dangerous = True
#             break
# if not not_dangerous:
#     print("NO")


# s = list(input())
# ans = ""
# while "+" in s:
#     s.remove("+")
# s.sort()
# for i in range(len(s)):
#     if i != len(s) - 1:
#         ans += s[i] + "+"
#     else:
#         ans += s[i]
#
# print(ans)


# s = list(input())
# capital_letter = s[0]
# s = s[1:]
# ans = ""
# capital_letter = str(capital_letter).capitalize()
# s = [capital_letter] + s
# print(ans.join(s))

# import math
# matrix = []
# for i in range(5):
#     matrix.append(list(map(int, input().split())))
#     if 1 in matrix[i]:
#         (x, y) = i, matrix[i].index(1)
#
# print(str(int(math.fabs(x - 2) + math.fabs(y - 2))))

# input()
# s = list(input())
# ans = []
# counter = 0
# for i in range(len(s) - 1):
#     if s[i] == s[i + 1]:
#         counter += 1
#     else:
#         ans.append(s[i + 1])
#
# print(counter)

# s = set(list(input()))
# print("CHAT WITH HER!" if len(s) % 2 == 0 else "IGNORE HIM!")

# input()
# groups = list(map(int, input().split()))
# group_type = [0, 0, 0, 0]
# ans = 0
# for i in groups:
#     group_type[i - 1] += 1
# ans += group_type[-1]
#
# group_type[-1] = 0
#
# ans += min(group_type[0], group_type[2])
#
# if group_type[0] < group_type[2]:
#     group_type[2] = group_type[2] - group_type[0]
#     group_type[0] = 0
# elif group_type[0] == group_type[2]:
#     group_type[0] = 0
#     group_type[2] = 0
# else:
#     group_type[0] = group_type[0] - group_type[2]
#     group_type[2] = 0
#
# ans += group_type[1] // 2
#
# group_type[1] = group_type[1] % 2
#
# if group_type[0]:
#     taxi = group_type[1] * 2
#     for i in range(group_type[0]):
#         if taxi == 4:
#             ans += 1
#             taxi = 0
#         taxi += 1
#
#     if taxi:
#         ans += 1
# else:
#     ans += group_type[2]
#     if group_type[1]:
#         ans += 1
#
# print(ans)

# n = int(input())
# minimum_capacity = 0
# io_list = []
# io_recorder = 0
# for i in range(n):
#     io_list.append(tuple(map(int, input().split())))
#     if i == 0:
#         minimum_capacity = io_list[0][1]
#     io_recorder -= io_list[i][0]
#     io_recorder += io_list[i][1]
#     if minimum_capacity < io_recorder:
#         minimum_capacity = io_recorder
# print(minimum_capacity)


# n, index = map(int, input().split())
# value_list = list(map(int, input().split()))
# solved = False
# pointer = 0
# for i in range(len(value_list) + 1):
#     if pointer == index - 1:
#         print("YES")
#         solved = True
#         break
#     if pointer < len(value_list):
#         pointer += value_list[pointer]
# if not solved:
#     print("NO")

# import math
#
# n = int(input())
# list1 = list(map(int, input().split()))
# m = int(input())
# list2 = list(map(int, input().split()))
#
# graph = dict()
#
# for i in list1:
#     graph[i] = []
# if n > m:
#     n, m = m, n
#     list1, list2 = list2, list1
#
# for i in list1:
#     for j in list2:
#         if math.fabs(i - j) == 1:
#             graph[i].append(j)
#
# value_list = sorted(list(graph.values()))
# # while graph:
# for i in value_list:
#

# k, n, w = map(int, input().split())
# cost = 0
# for i in range(w):
#     cost += (i + 1) * k
# if cost - n > 0:
#     print(cost - n)
# else:print("0")

# n = int(input())
# xi = 0
# yi = 0
# zi = 0
# for i in range(n):
#     xyz = list(map(int, input().split()))
#     xi += xyz[0]
#     yi += xyz[1]
#     zi += xyz[2]
# if xi == yi == zi == 0:
#     print("YES")
# else:
#     print("NO")

#
# import sys
# s = input()
# print("YES" if "H" in s or "Q" in s or "9" in s else (print("NO"), sys.exit()))

# s = input()
# is_changed = False
# if s.isupper():
#     print(s.lower())
#     is_changed = True
#
# if ord(s[0]) > 96 and (s[1:].isupper() or len(s) == 1):
#     print(s[0].upper() + s[1:].lower())
#     is_changed = True
# if not is_changed:False
#     print(s)


# ans = 0
# for i in range(int(input())):
#     p, q = map(int, input().split())
#     if q - p >= 2:
#         ans += 1
# print(ans)


# n, t = map(int, input().split())
# main_list = list(input())
# for i in range(t):
#     j = 0
#     while j < n:
#         if j != n - 1:
#             if main_list[j] == "B" and main_list[j + 1] == "G":
#                 main_list[j], main_list[j + 1] = main_list[j + 1], main_list[j]
#                 j += 1
#         j += 1
# print("".join(main_list))


# ans = 0
# number = list(map(int, list(input())))
# for i in number:
#     if i == 4 or i == 7:
#         ans += 1
#     if ans > 7:
#         break
# if ans == 7 or ans == 4:
#     print("YES")
# else:
#     print("NO")


# def odd_sum (n):
#     return ((1 + n)/2) * ((n // 2) + 1)
#
#
# def even_sum (n):
#     return ((2 + n) / 2) * (n // 2)
#
#
# n = int(input())
# if n % 2 == 1:
#     # print(int(-odd_sum(n) + even_sum(n - 1)))
#     print(odd_sum(n))
#
# else:
#
#     # print(int(-odd_sum(n - 1) + even_sum(n)))
#     print(even_sum(n))


# s1 = input()
# s2 = input()
# is_solved = False
# if len(s1) != len(s2):
#     is_solved = True
# if not is_solved:
#     for i in range(len(s1)):
#         if s1[i] != s2[len(s1) - 1 - i]:
#             is_solved = True
#             break
# if is_solved:
#     print("NO")
# else:
#     print("YES")

# n = int(input())
# first_time = True
# ans = 0
# income_list = input().split()
# for i in range(n):
#     j = 0
#     while i + j < n - 1:
#         if not (income_list[i + j] < income_list[i + j + 1]):
#             break
#         j += 1
#         # print(j)
#     if j > 0:
#         j += 1
#     if first_time:
#         ans = j
#         first_time = False
#     else:
#         if j > ans:
#             ans = j
#
# print(ans)
#


# n = int(input())
# step_list = input().split()
# is_running = True
# for i in range(n):
#
#
#     while is_running:
#         if temp > step_list[i + j]:
#             is_running = False
#         temp = step_list[i + j]

# import math
# for i in range(1000000000):
#     for j in range(10000000000):
#         print(i, j, i**j, j**i, math.factorial(i), math.factorial(j))

# length = int(input())
# input_string_list = list(map(int, input().split()))
# first_time = True
# temp_list = []
# ans = 0

# temp_list.append(1)
# for i in range(1, length):
#     temp = temp_list[-1]
#     if input_string_list[i] >= input_string_list[i - 1]:
#         temp_list.append(temp + 1)
#     else:
#         temp_list.append(1)

# print(max(temp_list))


# n = int(input())
# main_list = list(map(int, input().split()))
# main_dict = dict()
# ans = ""
# for i in range(1, len(main_list) + 1):
#     main_dict[main_list[i - 1]] = i
# # print(main_dict)
# for i in range(1, n + 1):
#     ans += str(main_dict[i]) + " "
# print(ans[:-1])

# length = int(input())
# input_string_list = list(map(int, input().split()))
# length_list = []
# length_list.append(1)
# for i in range(length - 1):
#     if input_string_list[i] <= input_string_list[i + 1]:
#         length_list[-1] += 1
#     else:
#         length_list.append(1)
# print(max(length_list))

# input_string = input()
# input_string_list = list(input_string)
# input_string_length = len(input_string_list)
# lower_count = 0
# for i in range(input_string_length):
#     if ord(input_string_list[i]) > 96:
#         lower_count += 1
# lower_count = float(lower_count)
# if lower_count >= input_string_length / 2:
#     print(input_string.lower())
# else:
#     print(input_string.upper())


def odd_sum(n):
    return (n + 1)*(n // 2 + 1) // 2


def even_sum(n):
    return (n + 2)*(n // 2) // 2


n = int(input())

if n % 2 == 0:
    print(even_sum(n) - odd_sum(n - 1))
else:
    print(even_sum(n - 1) - odd_sum(n))