#include<iostream>  
#include<algorithm>  
#include<stdio.h>  
#include<string.h>  
#include<math.h>  
#include<stack>
#include<queue>  
#define MAXN 2205
#define MAXM 25005  
#define oo 1000000007  
#define ll long long  
using namespace std;      
struct node
{
      int u,v,next; 
}edge[MAXM];
int ne,_next[MAXN],DfsIndex,tpnum,tp[MAXN],dfn[MAXN],low[MAXN]; 
void addedge(int u,int v)
{
      edge[++ne].next=_next[u],_next[u]=ne;
      edge[ne].u=u,edge[ne].v=v;
} 
bool instack[MAXN];
stack<int> S;
void tarjan(int x)
{
      dfn[x]=low[x]=++DfsIndex;
      instack[x]=true,S.push(x);
      for (int k=_next[x];k;k=edge[k].next)
      {
              int v=edge[k].v;
              if (!dfn[v])
              {
                     tarjan(v);
                     low[x]=min(low[x],low[v]);
              }else
              if (instack[v])
                     low[x]=min(low[x],dfn[v]);                           
      }
      if (dfn[x]==low[x])
      {
              tpnum++;
              do
              {
                     x=S.top(),S.pop();
                     instack[x]=false;
                     tp[x]=tpnum;
              }while (low[x]!=dfn[x]);
      }
}
bool color[MAXN];
bool judge(int n)
{
      for (int i=1;i<=n;i++)
      {
           if (tp[i<<1]==tp[i<<1|1]) return false;
           if (color[i<<1] && color[i<<1|1]) return false;
      }      
      return true;
} 
void dfs(int x)
{
      color[x]=1;
      for (int k=_next[x];k;k=edge[k].next)
         if (!color[edge[k].v]) dfs(edge[k].v);
}
int main() 
{          
      int n,m,u,v,i; 
      while (~scanf("%d%d",&n,&m))
      {
              ne=0,memset(_next,0,sizeof(_next));
              while (m--)
              {
                      scanf("%d%d",&u,&v);
                      if (u<0) u=(-u)<<1;
                          else u=u<<1|1;
                      if (v<0) v=(-v)<<1;
                          else v=v<<1|1; 
                      addedge(u^1,v),addedge(v^1,u);
              }
              memset(dfn,0,sizeof(dfn));
              memset(instack,false,sizeof(instack));
              while (!S.empty()) S.pop();
              tpnum=DfsIndex=0;
              for (i=2;i<=(n<<1|1);i++)
                 if (!dfn[i]) tarjan(i); 
              memset(color,false,sizeof(color));
              dfs(3);
              if (!judge(n)) printf("no\n");
                        else printf("yes\n");                        
      }
      return 0;
}