#include <iostream>
#include <cstdio>
#include <bitset>
#include <vector>
#include <algorithm>
using namespace std;
const int SIZE=1005;

struct RNM{
    
    int a,b,c,d;
    
    RNM(int a=0,int b=0,int c=0,int d=0):a(a),b(b),c(c),d(d){}
    
};

vector<RNM> ans1,ans2;
bitset<SIZE> g[SIZE],h[SIZE];
int deg1[SIZE],deg2[SIZE];
int n;

bool check(int x){
    
    if(g[x]==h[x])
    {
        g[x].reset();
        h[x].reset();
        return false;
    }
    
    int u=(g[x]^(g[x]&h[x]))._Find_first();
    int v=(h[x]^(g[x]&h[x]))._Find_first();
    
    auto f1=(g[v]^(g[u]&g[v]));f1[u]=0;
    auto f2=(h[u]^(h[u]&h[v]));f2[v]=0;
    int t1=f1._Find_first();
    int t2=f2._Find_first();
    
    if(t1<=n)
    {
        ans1.emplace_back(x,u,v,t1);
        g[x][u]=g[u][x]=0;
        g[x][v]=g[v][x]=1;
        g[t1][v]=g[v][t1]=0;
        g[t1][u]=g[u][t1]=1;
    }
    else
    {
        ans2.emplace_back(x,u,v,t2);
        h[x][v]=h[v][x]=0;
        h[x][u]=h[u][x]=1;
        h[t2][u]=h[u][t2]=0;
        h[t2][v]=h[v][t2]=1;
    }
    
    return true;
    
}

int main(){
    
    int m;
    cin>>n>>m;
    
    for(int i=1;i<=m;i++)
    {
        int u,v;
        cin>>u>>v;
        g[u][v]=g[v][u]=true;
        deg1[u]++,deg1[v]++;
    }
    
    for(int i=1;i<=m;i++)
    {
        int u,v;
        cin>>u>>v;
        h[u][v]=h[v][u]=true;
        deg2[u]++,deg2[v]++;
    }
    
    for(int i=1;i<=n;i++)
        if(deg1[i]!=deg2[i])    
            return puts("-1"),0;
            
    for(int i=1;i<=n;i++)
        while(check(i));
        
    reverse(ans2.begin(),ans2.end());
    cout<<ans1.size()+ans2.size()<<'\n';
    
    for(auto v:ans1) cout<<v.a<<" "<<v.b<<" "<<v.c<<" "<<v.d<<'\n';
    for(auto v:ans2) cout<<v.a<<" "<<v.b<<" "<<v.c<<" "<<v.d<<'\n';
    
    return 0;
        
    
}