#include<bits/stdc++.h>
using namespace std;
bitset<1005>b1[1005],b2[1005];
int d1[1005],d2[1005];
vector<pair<pair<int,int>,pair<int,int>>>ans1,ans2;
void dstt1(int a,int b,int c,int d){
	ans1.emplace_back(make_pair(a,b),make_pair(c,d));
	b1[a][b]=0,b1[a][c]=1;
	b1[b][a]=0,b1[b][d]=1;
	b1[c][d]=0,b1[c][a]=1;
	b1[d][c]=0,b1[d][b]=1;
}
void dstt2(int a,int b,int c,int d){
	ans2.emplace_back(make_pair(a,b),make_pair(c,d));
	b2[a][b]=1,b2[a][c]=0;
	b2[b][a]=1,b2[b][d]=0;
	b2[c][d]=1,b2[c][a]=0;
	b2[d][c]=1,b2[d][b]=0;
}
int main(){
	int n,m;
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;++i){
		int u,v;
		scanf("%d%d",&u,&v);
		b1[u].set(v),b1[v].set(u);
		++d1[u],++d1[v];
	}
	for(int i=1;i<=m;++i){
		int u,v;
		scanf("%d%d",&u,&v);
		b2[u].set(v),b2[v].set(u);
		++d2[u],++d2[v];
	}
	int flag=1;
	for(int i=1;i<=n;++i)if(d1[i]!=d2[i])flag=0;
	if(!flag){
		puts("-1");
		return 0;
	}
	bitset<1005>bb;
	for(int i=1;i<=n;++i)bb.set(i);
	while(bb.count()){
		int x=bb._Find_first();
		auto at=b1[x]&b2[x];
		auto a1=b1[x]^at,a2=b2[x]^at;
		if(!a1.count()){
			for(int i=1;i<=n;++i)b1[x][i]=b1[i][x]=0;
			for(int i=1;i<=n;++i)b2[x][i]=b2[i][x]=0;
			bb[x]=0;
			continue;
		}
		int u=a1._Find_first(),v=a2._Find_first();
		auto aa=b1[v]^(b1[u]&b1[v]);
		if(aa.count()){
			int t=aa._Find_first();
			dstt1(x,u,v,t);
		}else{
			auto aaa=b1[u]^(b1[u]&b1[v]);
			aaa.reset(x);
			int t=aaa._Find_first();
			dstt2(x,u,v,t);
		}
	}
	int s1=ans1.size(),s2=ans2.size();
	printf("%d\n",s1+s2);
	for(int i=0;i<s1;++i){
		auto pp=ans1[i];
		auto p1=pp.first,p2=pp.second;
		printf("%d %d %d %d\n",p1.first,p1.second,p2.first,p2.second);
	}
	for(int i=s2-1;i>=0;--i){
		auto pp=ans2[i];
		auto p1=pp.first,p2=pp.second;
		printf("%d %d %d %d\n",p1.first,p1.second,p2.first,p2.second);
	}
	return 0;
}