題目描述

Given an interger n, try to find the sum of the sequence with following pattern:2-3 + 4-5 + 6 +...n

輸入格式

An integer n

n >= 2

輸出格式

an integer represent the answer

Samples

["4","6","5"]

["3","4","-2"]

備註

試試不用循環結構吧！

原始資料

• Zero1 題號：a154
• Hydro 題號：Z0154
• Locale：zh_TW
• Display：open