#include<bits/stdc++.h>
namespace my_std{
    using namespace std;
    #define pii pair<int,int>
    #define fir first
    #define sec second
    #define MP make_pair
    #define rep(i,x,y) for (int i=(x);i<=(y);i++)
    #define drep(i,x,y) for (int i=(x);i>=(y);i--)
    #define go(x) for (int i=head[x];i;i=edge[i].nxt)
    #define templ template<typename T>
    #define sz 1111
    typedef long long ll;
    typedef double db;
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    templ inline T rnd(T l,T r) {return uniform_int_distribution<T>(l,r)(rng);}
    templ inline bool chkmax(T &x,T y){return x<y?x=y,1:0;}
    templ inline bool chkmin(T &x,T y){return x>y?x=y,1:0;}
    templ inline void read(T& t)
    {
        t=0;char f=0,ch=getchar();double d=0.1;
        while(ch>'9'||ch<'0') f|=(ch=='-'),ch=getchar();
        while(ch<='9'&&ch>='0') t=t*10+ch-48,ch=getchar();
        if(ch=='.'){ch=getchar();while(ch<='9'&&ch>='0') t+=d*(ch^48),d*=0.1,ch=getchar();}
        t=(f?-t:t);
    }
    template<typename T,typename... Args>inline void read(T& t,Args&... args){read(t); read(args...);}
    char __sr[1<<21],__z[20];int __C=-1,__zz=0;
    inline void Ot(){fwrite(__sr,1,__C+1,stdout),__C=-1;}
    inline void print(int x)
    {
        if(__C>1<<20)Ot();if(x<0)__sr[++__C]='-',x=-x;
        while(__z[++__zz]=x%10+48,x/=10);
        while(__sr[++__C]=__z[__zz],--__zz);__sr[++__C]='\n';
    }
    void file()
    {
        #ifdef NTFOrz
        freopen("a.in","r",stdin);
        #endif
    }
    inline void chktime()
    {
        #ifdef NTFOrz
        cerr<<clock()/1000.0<<'\n';
        #endif
    }
    #ifdef mod
    ll ksm(ll x,int y){ll ret=1;for (;y;y>>=1,x=x*x%mod) if (y&1) ret=ret*x%mod;return ret;}
    ll inv(ll x){return ksm(x,mod-2);}
    #else
    ll ksm(ll x,int y){ll ret=1;for (;y;y>>=1,x=x*x) if (y&1) ret=ret*x;return ret;}
    #endif
}
using namespace my_std;

int n,m;
bitset<sz>e1[sz],e2[sz];
int deg[sz];

vector<pair<pii,pii>>ans1,ans2;

void flip1(int a,int b,int c,int d)
{
    ans1.push_back(MP(MP(a,b),MP(c,d)));
    e1[a].flip(b),e1[b].flip(a);
    e1[a].flip(c),e1[c].flip(a);
    e1[c].flip(d),e1[d].flip(c);
    e1[b].flip(d),e1[d].flip(b);
}
void flip2(int a,int b,int c,int d)
{
    ans2.push_back(MP(MP(a,b),MP(c,d)));
    e2[a].flip(b),e2[b].flip(a);
    e2[a].flip(c),e2[c].flip(a);
    e2[c].flip(d),e2[d].flip(c);
    e2[b].flip(d),e2[d].flip(b);
}

int main()
{
    file();
    read(n,m);
    int x,y;
    rep(i,1,m) read(x,y),e1[x][y]=e1[y][x]=1,++deg[x],++deg[y];
    rep(i,1,m) read(x,y),e2[x][y]=e2[y][x]=1,--deg[x],--deg[y];
    rep(i,1,n) if (deg[i]!=0) return puts("-1"),0;
    bitset<sz>W; rep(i,1,n) W[i]=1;
    rep(i,1,n)
    {
        W[i]=0;
        vector<int>V1,V2;
        rep(j,i+1,n)
            if (e1[i][j]&&!e2[i][j]) V1.push_back(j);
            else if (e2[i][j]&&!e1[i][j]) V2.push_back(j);
        rep(j,0,(int)V1.size()-1)
        {
            int x3=V1[j],x2=V2[j];
            bitset<sz>B=e1[x2]&~e1[x3]&W; B[x3]=0;
            if (B.count())
            {
                int z=B._Find_first();
                flip1(i,x3,x2,z);
            }
            else
            {
                B=e2[x3]&~e2[x2]&W; B[x2]=0;
                assert(B.count());
                int z=B._Find_first();
                flip2(i,x2,x3,z);
            }
        }
        assert(e1[i]==e2[i]);
    }
    printf("%d\n",(int)ans1.size()+(int)ans2.size());
    for (auto [p1,p2]:ans1) printf("%d %d %d %d\n",p1.first,p1.second,p2.first,p2.second);
    reverse(ans2.begin(),ans2.end());
    for (auto [p1,p2]:ans2) printf("%d %d %d %d\n",p1.first,p2.first,p1.second,p2.second);
    return 0;
}