#include<bits/stdc++.h>
using namespace std;
const int N = 1005;
int n , m ;
bitset<N>g[N],h[N];
int deg[N];

struct Ans{int a,b,c,d;};
vector<Ans>pre,suf;


void change(int opt,int a,int b,int c,int d){
    if(opt==0) pre.push_back({a,b,c,d});
    else suf.push_back({a,b,c,d});
    assert(g[a][b]==1);
    assert(g[c][d]==1);
    assert(g[a][c]==0);
    if(opt==0){
        g[a][b] = g[b][a] = 0 ;
        g[c][d] = g[d][c] = 0 ;
        g[a][c] = g[c][a] = 1;
        g[b][d] = g[d][b] = 1;
    }else {
         h[a][b] = h[b][a] = 0 ;
        h[c][d] = h[d][c] = 0 ;
        h[a][c] = h[c][a] = 1;
        h[b][d] = h[d][b] = 1;  
    }

}

int main()
{
    cin >> n >> m;
    for(int i=1;i<=m;i++){
        int u,v;cin >> u >> v;
        g[u].set(v,1);
        g[v].set(u,1);
        deg[u] ++ ;deg[v] ++ ;
    }
    for(int i=1;i<=m;i++){
        int u,v;cin >> u >> v;
        h[u].set(v,1);
        h[v].set(u,1);
        deg[u] -- ;deg[v] -- ;
    }

    for(int i=1;i<=n;i++) if(deg[i]!=0){
        cout << -1;
        return 0;
    }
        // asdasdasd

    for(int i=1;i<=n;i++){
        vector<int>gg,hh;
        for(int j=i+1;j<=n;j++)
            if(g[i][j]!=h[i][j]){
                if(g[i][j]) gg.emplace_back(j);
                else hh.emplace_back(j);
            }
        for(int j=0;j<gg.size();j++){
            int v = gg[j];
            int w = hh[j];
            bitset<N>tmp = (g[w] ^ g[v]) & g[w];
            int t = tmp._Find_first();
            while(t==v || t==w) t=tmp._Find_next(t);

            if(t==tmp.size()){
                tmp = (h[w] ^ h[v]) & h[v];
                t = tmp._Find_first();
                while(t==v || t==w) t=tmp._Find_next(t);
                change(1,i,w,v,t); 
            }else {
                change(0,i,v,w,t);
            }
        }
    }
    reverse(suf.begin(),suf.end());
    cout << pre.size() + suf.size() << '\n';
    for(auto v:pre)
        cout << v.a << ' ' << v.b << ' ' << v.c << ' ' << v.d << '\n';
    for(auto v:suf)
        cout << v.a << ' ' << v.c << ' ' << v.b << ' ' << v.d << '\n';

    

    return 0;
}