C++ language

Write a C++ program that prompts the user to input an integer (n) and outputs the sum of all integers i from 1 to n, but if i = 2 ( is an integer) then i will have a negative sign. For example, for n= the sum is equal to -1-2+3-4 + 5 +6+7 -8= 6, because 1, 2, 4 and 8 are 2", 2", 2 and 2 respectively while 3, 5, 6, and 7 are not equal to 2 for any integer j. Note: Solve this question without defining any other function apart from the main function