Idol

Sanggeun is competing in the preliminary round of an idol audition program.

In this audition, every contestant gets a chance to show off their charm in front of the judges. After all the performances are finished, the judges gather and vote. Each judge must cast exactly two votes, and each vote is one of the following two kinds.

• A "for" vote: the judge wants that contestant to advance to the next round.
• An "against" vote: the judge wants that contestant to be eliminated in this round.

A single judge may cast two "for" votes, two "against" votes, or one "for" and one "against". However, a single judge always votes on two different contestants.

The number of contestants who advance is not fixed in advance. It is possible that nobody advances, and it is possible that everybody advances.

Sanggeun wants to arrange the list of advancing contestants so that he advances to the next round, without any judge becoming suspicious of the result. Each judge believes that at least one of the two votes they cast must be reflected in the result. In other words, if both of a judge's votes contradict the result, that judge becomes suspicious.

For example, suppose a judge cast a "for" vote on contestant A and an "against" vote on contestant B. If, in the final list, A is eliminated and B advances, then neither vote was reflected, so that judge becomes suspicious.

Given how every judge voted, write a program that decides whether it is possible to build a list of advancing contestants that includes Sanggeun while no judge becomes suspicious. Sanggeun is contestant number 1.

The input consists of several test cases. Process each test case until the end of the file.

The first line of each test case contains the number of contestants $n$ ($2 \le n < 1000$) and the number of judges $m$ ($1 \le m < 2000$).

Each of the next $m$ lines contains two integers $a$ and $b$ describing the two votes cast by one judge ($1 \le |a|, |b| \le n$, $|a| \ne |b|$). If a value $x$ is negative, that judge cast an "against" vote on contestant $|x|$; if it is positive, the judge cast a "for" vote on contestant $|x|$.

Contestants are numbered from 1 to $n$, and Sanggeun is contestant number 1.

For each test case, print yes on its own line if it is possible to build a list of advancing contestants that includes Sanggeun without making any judge suspicious, and no otherwise.