1466. Excursion

输入格式

The first line of the input contains two positive integers $n$ and $m$ ($1\le n\le 20000,1\le m\le 8000$); $n$ is the number of travelers, and $m$ is the number of cities. The travelers are numbered from $1$ to $n$, and the cities are numbered from $1$ to $m$. Each of the following $n$ lines contains two nonzero integers, separated by single space. The $i+1$-th line contains numbers $w_i$ and $v_i$ representing wishes of the $i$-th traveler, $-m\le w_i,v_i\le m,w_i,v_i\ne 0$. Positive number means that the traveler wishes to visit that city, and negative number means that the traveler does not wish to visit the city specified by the absolute value of the number.

输出格式

Your program should write one nonnegative integer $l$, the number of cities to be visited, in the first line of the output. In the second line, there should be written exactly $l$ positive integers in the ascending order, representing cities to be visited.

In case it is not possible to form such a list of cities, your program should write in the first and only line the word NO.

3 4
1 -2
2 4
3 1

4
1 2 3 4