2-sat

#include<vector>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
using namespace std;
#define pb push_back
const int M=5000;
vector<int>g[M];
int low[M],dfn[M],sta[M],vis[M],cmp[M];
int tot,cnt,top;
void tarjan(int u){
    low[u]=dfn[u]=++cnt;
    sta[++top]=u;
    vis[u]=1;
    for(int i=0;i<g[u].size();i++){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        else if(vis[v])
            low[u]=min(low[u],dfn[v]);
    }
    if(low[u]==dfn[u]){
        tot++;
        cmp[u]=tot;
        vis[u]=0;
        while(sta[top]!=u){
            cmp[sta[top]]=tot;
            vis[sta[top--]]=0;
        }
        top--;
    }
}
int main(){
    int n,m;
    while(~scanf("%d%d",&n,&m)){
        for(int i=0;i<=2*n;i++)
            g[i].clear(),low[i]=dfn[i]=cmp[i]=vis[i]=sta[i]=0;
        top=cnt=tot=0;
        while(m--){
            int a1,a2,c1,c2;
            scanf("%d%d%d%d",&a1,&a2,&c1,&c2);
            int a = a1*2+c1;
            int b = a2*2+c2;
            g[a].pb(b^1);
       //     cout<<u<<"----"<<2*a2+(1-c2)<<endl;
      //      g[2*a2+(1-c2)].pb(u);
            g[b].pb(a^1);
       //     cout<<v<<"----"<<2*a1+(1-c1)<<endl;
      //      g[2*a1+(1-c1)].pb(v);
        }
        for(int i=0;i<2*n;i++)
            if(!dfn[i])
                tarjan(i);
        int flag=1;
        for(int i=0;i<n;i++){
            if(cmp[2*i]==cmp[2*i+1]){
                flag=0;
                break;
            }

        }
        if(flag)
            puts("YES");
        else
            puts("NO");

    }

    return 0;
}