KPSUM - The Sum

## 题面 给你一个正整数 $N$，依次把 $1$ 到 $N$ 写下来，在每两个数字之间交替的加上加减号，求这个表达式的值。例如，$N=15$ 时，表达式为 $1-2+3-4+5-6+7-8+9-1+0-1+1-1+2-1+3-1+4-1+5$。 $1 \le N \le 10^{15}$。

One of your friends wrote numbers 1, 2, 3, ..., N on the sheet of paper. After that he placed signs + and - between every pair of adjacent digits alternately. Now he wants to find the value of the expression he has made. Help him. For example, if N=12 then +1 -2 +3 -4 +5 -6 +7 -8 +9 -1+0 -1+1 -1+2 = 5

Each line contains one integer number N (1≤ N ≤ 10 $ ^{15} $ ). Last line contains 0 and shouldn't be processed. Number of lines in the input does not exceed 40.

For every line in the input write the answer on a separate line.

12
0

5