#include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<algorithm>
#include<cmath>
#include<climits>
#include<string>
#include<set>
#include<map>
#include<iostream>
using namespace std;
#define rep(i,n) for(int i=0;i<((int)(n));i++)
#define reg(i,a,b) for(int i=((int)(a));i<=((int)(b));i++)
#define irep(i,n) for(int i=((int)(n))-1;i>=0;i--)
#define ireg(i,a,b) for(int i=((int)(b));i>=((int)(a));i--)
typedef long long int lli;
typedef pair<int,int> mp;
#define fir first
#define sec second
#define IINF INT_MAX
#define LINF LLONG_MAX
#define eprintf(...) fprintf(stderr,__VA_ARGS__)
#define pque(type) priority_queue<type,vector<type>,greater<type> >
#define memst(a,b) memset(a,b,sizeof(a))
#define iter(v,ite) for(auto ite=(v).begin();ite!=(v).end();ite++)
#define mimunum(v,x) distance((v).begin(),lower_bound((v).begin(),(v).end(),x))

struct makehtml{
	vector<string> ss;
	
	makehtml(){}
	/*
	ctx.beginPath()
	ctx.moveTo(50.000000,100.000000);
	ctx.lineTo(150.000000,100.000000);
	ctx.stroke();
	
	ctx.fillStyle = "rgb(0,0,0)";
	
	ctx.beginPath();
	ctx.arc(168,59,3,0,360);
	ctx.fill();

	*/
	
	void out(const char* name,double mag){
		char fname[100];
		sprintf(fname,"%s.html",name);
		FILE* fp = fopen(fname,"w");
		if(fp==NULL){
			eprintf("can't open %s\b",fname);
			exit(-1);
		}
		fprintf(fp,"<html>\n<head>\n<title>");
		fprintf(fp,"%s",name);
		fprintf(fp,"</title>\n</head>\n<body>\n");
		fprintf(fp,"<p>name: %s</p>",name);
		fprintf(fp,"<canvas id=\"vis\" width=\"%d\" height=\"%d\"></canvas>",2000,2000);
		fprintf(fp,"<script>\n");
		fprintf(fp,"var vis = document.getElementById('vis');\nvar ctx = vis.getContext('2d');\n");
		
		rep(i,ss.size()){
			fprintf(fp,"%s\n",ss[i].c_str());
		}
		
		fprintf(fp,"</script>\n");
		fprintf(fp,"</body>\n</html>\n");
		fclose(fp);
	}

} makehtml_instance;


void addhtml(string s){
	makehtml_instance.ss.push_back(s);
}

void outhtml(string s,double mag){
	makehtml_instance.out(s.c_str(),mag);
}

void randcolhtml(){
	char ns[100];
	sprintf(ns,"ctx.fillStyle = \"rgb(%d,%d,%d)\";",rand()%128,rand()%128,rand()%128);
	addhtml(ns);
	sprintf(ns,"ctx.strokeStyle = \"rgb(%d,%d,%d)\";",rand()%128,rand()%128,rand()%128);
	addhtml(ns);
}

void inithtml(){
	makehtml_instance.ss.clear();
}


const double pi=3.14159265358979;

struct pos{
	double x,y;
	double norm,norm2;
	pos tov(pos a){
		return (a-(*this));
	}
	pos operator+(pos a)const{
		pos res=a;
		res.x+=x; res.y+=y;
		res=pos(res.x,res.y);
		return res;
	}
	pos operator-(pos a)const{
		pos res=(*this);
		res.x-=a.x; res.y-=a.y;
		res=pos(res.x,res.y);
		return res;
	}
	pos scalar(double a){
		pos res=(*this);
		res.x*=a; res.y*=a;
		res=pos(res.x,res.y);
		return res;
	}
	double dot(pos a){
		return x*a.x + y*a.y;
	}
	double cross(pos a){
		return x*a.y - y*a.x;
	}
	double theta(pos a){
		double nt=dot(a)/(norm*a.norm);
		nt=acos(max(-1.0,min(1.0,nt)));
		double ns=(cross(a)/(norm*a.norm));
		if(ns<0)nt*=-1;
		return nt;
	}
	pos(double ix,double iy){
		x=ix; y=iy;
		norm2=x*x+y*y;
		norm=sqrt(norm2);
	}
	pos(){}
	string str(){
		char ns[50];
		sprintf(ns,"(%lf %lf)",x,y);
		return	string(ns);
	}
	void outh(double mag){
		char ns[50];
		addhtml("ctx.beginPath();"); 
		sprintf(ns,"ctx.arc(%lf,%lf,%lf,0,360);",x*mag,y*mag,5.0); addhtml(ns);
		addhtml("ctx.fill();");
	}
	static pos polar(double r,double t){
		return pos(r*cos(t),r*sin(t));
	}
	pos turn(double t){
		//t[rad]??????
		return pos(x*cos(t)-y*sin(t),x*sin(t)+y*cos(t));
	}
	pos tolen(double a){
		//?????????a?????????
		return this->scalar(a/norm);
	}
	bool  operator<(pos a) const{
		//if(x!=a.x)return (x<a.x);
		//return (y<a.y);
		if(y!=a.y)return (y<a.y);
		return (x<a.x);
	}	
	bool operator>(pos a)const{ return (a<(*this)); }	
	bool operator==(pos a)const{ return (x==a.x && y==a.y); }	
	bool operator!=(pos a)const{ return !(a==(*this)); }	
};
struct line{
	pos p,q;
	pos vec;
	string str(){
		return ("("+p.str()+"-"+q.str()+")");
	}
	void outh(double mag){
		char ns[50];
		addhtml("ctx.beginPath();");
		sprintf(ns,"ctx.moveTo(%lf,%lf);",p.x*mag,p.y*mag); addhtml(ns);
		sprintf(ns,"ctx.lineTo(%lf,%lf);",q.x*mag,q.y*mag); addhtml(ns);
		addhtml("ctx.stroke();");
	}
	bool iscross(line a){
		double da,db;
		da=vec.cross( p.tov(a.p) );
		db=vec.cross( p.tov(a.q) );
		if(da*db>=0)return false;
		da=a.vec.cross( a.p.tov(p) );
		db=a.vec.cross( a.p.tov(q) );
		return (da*db<0);
	}
	line(pos ip,pos iq){
		p=ip; q=iq;
		vec=p.tov(q);
	}
	line(){}
	double dist(pos a){
		if(p.tov(a).dot( p.tov(q) )<0)return p.tov(a).norm;
		if(q.tov(a).dot( q.tov(p) )<0)return q.tov(a).norm;
		return abs(vec.cross(p.tov(a))/vec.norm);
	}
	double dist(line a){
		if(iscross(a))return 0;
		return min( min(dist(a.p),dist(a.q)) , min(a.dist(this->p),a.dist(this->q)));
	}
	pos crosspos(line a){
		/*
		double d1=a.vec.cross(a.p.tov(p))/a.vec.norm;
		double d2=a.vec.cross(a.p.tov(q))/a.vec.norm;
		bool isc=(d1*d2<0);
		d1=abs(d1); d2=abs(d2);
		double sc;
		if(!isc){
			if(d1>d2)sc = d1/(d1-d2);
			else sc=d1/(d2-d1);
		}
		else sc = d1/(d1+d2);
		return (p+vec.scalar(sc));*/
		
		double t = a.vec.cross(a.p-p)/a.vec.cross(vec);
		return (p+vec.scalar(t));
	}
	line bisection(){//?????´????????????
		//???????¨??????????90????????£???????????????
		pos fr=(p+q).scalar(0.5);
		pos to=fr+pos(-vec.y,vec.x);
		return line(fr,to);
	}
	bool  operator<(line a) const{
		//if(x!=a.x)return (x<a.x);
		//return (y<a.y);
		if(p!=a.p)return (p<a.p);
		return (q<a.q);
	}	
	bool operator>(line a)const{ return (a<(*this)); }	
};

double lowofcos_getcos(double a,double b,double c){
	return ((a*a+b*b-c*c)/(2*a*b));
}
struct circle{
	pos p;
	double r;
	line koten(circle a){
		pos cv=p.tov(a.p);
		double br=atan2(cv.y,cv.x);
		double dr=acos(lowofcos_getcos( r, cv.norm, a.r));
		pos p1= p+pos::polar(r,br+dr);
		pos p2= p+pos::polar(r,br-dr);
		if(p1>p2)swap(p1,p2);
		return line(p1,p2);
	}
	circle(pos ip,double ir){
		p=ip; r=ir;
	}
	circle(){}
	void outh(double mag){
		char ns[50];
		addhtml("ctx.beginPath();"); 
		sprintf(ns,"ctx.arc(%lf,%lf,%lf,0,360);",p.x*mag,p.y*mag,r*mag); addhtml(ns);
		addhtml("ctx.stroke();");
	}
	
	vector<line> tangs(circle a){ //??\?????????
		//validate AOJ 2201
		circle b(*this);
		if(a.r<b.r)swap(a,b);
		vector<line> res;
		if((a.p-b.p).norm-(a.r-b.r)<0.0)return res;
		if((a.p-b.p).norm-(a.r-b.r)<1e-6){
			if((b.p-a.p).norm<1e-6)return res; //??¨???????????????
			pos rv=(b.p-a.p).tolen(a.r);
			res.push_back(line(rv+a.p,rv+a.p+rv.turn(pi/2.0)));
			return res;
		}
		{
			pos cv=b.p-a.p;
			double th=acos((a.r-b.r)/cv.norm);
			pos p1 = cv.turn(th);
			res.push_back(line(a.p+p1.tolen(a.r),b.p+p1.tolen(b.r)));
			pos p2 = cv.turn(-th);
			res.push_back(line(a.p+p2.tolen(a.r),b.p+p2.tolen(b.r)));
		}
		if((a.p-b.p).norm-(a.r+b.r)<0.0)return res;
		if((a.p-b.p).norm-(a.r+b.r)<1e-6){
			pos rv=(b.p-a.p).tolen(a.r);
			res.push_back(line(rv+a.p,rv+a.p+rv.turn(pi/2.0)));
			return res;
		}
		{
			pos cv=b.p-a.p;
			double th=acos((a.r+b.r)/cv.norm);
			pos p1 = cv.turn(th);
			res.push_back(line(a.p+p1.tolen(a.r),b.p+p1.tolen(-b.r)));
			pos p2 = cv.turn(-th);
			res.push_back(line(a.p+p2.tolen(a.r),b.p+p2.tolen(-b.r)));
		}
		return res;
	}
};

struct polygon{
	vector<pos> ps;
	polygon(){}
	void outh(double mag){
		char ns[50];
		addhtml("ctx.beginPath();");
		int ls=ps.size();
		sprintf(ns,"ctx.moveTo(%lf,%lf);",ps[ls-1].x*mag,ps[ls-1].y*mag); addhtml(ns);
		rep(i,ps.size()){
			sprintf(ns,"ctx.lineTo(%lf,%lf);",ps[i].x*mag,ps[i].y*mag); addhtml(ns);
		}
		addhtml("ctx.stroke();");
	}
	double area(){
		double res=0;
		rep(i,ps.size()){
			pos no=ps[i],to=ps[(i+1)%ps.size()];
			res+=(no.x+to.x)*(no.y-to.y);
		}
		if(res<0)reverse(ps.begin(),ps.end());
		return abs(res)/2;
	}
	polygon(pos a,pos b){
		ps.push_back(a);
		ps.push_back(pos(a.x,b.y));
		ps.push_back(b);
		ps.push_back(pos(b.x,a.y));
	}
	line gete(int a){
		a%=ps.size();
		return line(ps[a],ps[(a+1)%ps.size()]);
	}
	int isin(pos a){//?????´??????2,?????£????????????1,?????????0
		double as=0;
		rep(i,ps.size()){
			line ne=gete(i);
			if(ne.dist(a)<=0)return 1;
			pos v1=a.tov(ne.p);
			pos v2=a.tov(ne.q);
			as+=v1.theta(v2);
		}
		if(abs(as)<1e-6)return 0;
		return 2;
	}
	polygon convex_full(){
		polygon res;
		int ns=ps.size();
		vector<pos> vs(2*ns);
		sort(ps.begin(),ps.end());
		int k=0;
		rep(i,ns){
			pos np=ps[i];
			while(k>1 && vs[k-2].tov(vs[k-1]).cross(vs[k-1].tov(np))<-1e-6)k--;
			vs[k]=np; k++;
		}
		int t=k;
		ireg(i,0,ns-2){
			pos np=ps[i];
			while(k>t && vs[k-2].tov(vs[k-1]).cross(vs[k-1].tov(np))<-1e-6)k--;
			vs[k]=np; k++;
		}
		vs.resize(k-1);
		swap(vs,res.ps);
		return res;
	}
	polygon cut(line a){//???????????????????????????
		polygon res;
		rep(i,ps.size()){
			line ne=gete(i);
			double d1=a.vec.cross(a.p.tov(ne.p));
			double d2=a.vec.cross(a.p.tov(ne.q));
			if(d1<1e-6)res.ps.push_back(ne.p); //?????£????????£???eps
			if(d1*d2<-1e-6)res.ps.push_back(a.crosspos(ne)); //?????£????????????eps
		}
		return res;
	}
	
	vector<polygon> voronoi(vector<pos> ps){
		//validate AOJ 2160
		vector<polygon> res;
		rep(i,ps.size()){
			polygon np(*this);
			rep(j,ps.size()){
				if(i==j)continue;
				line nl = line(ps[j],ps[i]).bisection();
				
				np = np.cut(nl);
			}
			res.push_back(np);
		}
		return res;		
	}
	polygon mult(polygon a){
		return a;
	}
};

struct unifo{
	int n;
	int uni[100005];
	void init(int in){
		n=in;
		rep(i,n)uni[i]=i;
	}
	int find(int p){
		if(uni[p]==p)return p;
		else return (uni[p]=find(uni[p]));
	}
	void mer(int p,int q){
		if(!same(p,q))uni[find(p)]=find(q);
	}
	bool same(int p,int q){
		return find(p)==find(q);
	}
	void out(){
		vector<mp> ps;
		//printf("%d %d\n",i,find(i));
		rep(i,n)ps.push_back(mp(find(i),i));
		sort(ps.begin(),ps.end());
		printf("uni .. %d elem\n{ ",n);
		rep(i,n){
			if(i!=0 && ps[i-1].fir!=ps[i].fir)printf("}\n{ ");
			printf("%d ",ps[i].sec);
		}
		printf("}\n\n");
	}
} uni;



int n,m;
pos ps[1005];
pos qs[1005];

double ds[1005][1005];
double dist[1005];
int gone[1005];

int main(void){
	scanf("%d%d",&n,&m);
	rep(i,n+m){
		double x,y;
		scanf("%lf%lf",&x,&y);
		if(i<n)ps[i]=pos(x,y);
		else qs[i-n]=pos(x,y);
	}
	
	double ans=1e15;
	rep(qqq,2){
		line ne(ps[0],ps[1]);
		double ns=ne.vec.norm;
		
		uni.init(m+5);
		typedef pair<double,mp> dmp;
		vector<dmp> es;
		
		rep(i,m)dist[i]=-1;
		rep(i,m){
			rep(j,i){
				line ce(qs[i],qs[j]);
				ds[i][j]=ds[j][i]=(ce.iscross(ne)?-1:ce.vec.norm);
			}
		}
		/*
		rep(i,m)rep(j,m){
			printf("%d %d %lf\n",i,j,ds[i][j]);
		}*/
		memset(gone,0,sizeof(gone));
		dist[0]=0;
		
		
		for(;;){
			int nv=-1;
			rep(i,m){
				if(gone[i] || dist[i]<0)continue;
				if(nv==-1 || dist[i]<dist[nv])nv=i;
			}
			if(nv==-1){
				ns=1e15;
				break;
			}
			if(nv==1){
				ns+=dist[1];
				break;
			}
			gone[nv]=1;
			rep(j,m){
				if(gone[j] || ds[nv][j]<0)continue;
				double nc=dist[nv]+ds[nv][j];
				if(dist[j]<0 || nc<dist[j])dist[j]=nc;
			}
		}
		ans=min(ans,ns);
		swap(n,m);
		swap(ps,qs);
	}
	if(ans>1e10)printf("-1\n");
	else printf("%.11f\n",ans);
	return 0;
}