// Enjoy your stay.

#include <bits/stdc++.h>

#define EPS 1e-9
#define INF 1070000000LL
#define MOD 1000000007LL
#define fir first
#define foreach(it,X) for(auto it=(X).begin();it!=(X).end();it++)
#define ite iterator
#define mp make_pair
#define mt make_tuple
#define rep(i,n) rep2(i,0,n)
#define rep2(i,m,n) for(int i=m;i<(n);i++)
#define pb push_back
#define sec second
#define sz(x) ((int)(x).size())

using namespace std;

typedef istringstream iss;
typedef long long ll;
typedef pair<ll,ll> pi;
typedef stringstream sst;
typedef vector<ll> vi;

// geometry library from hos

const double PI = acos(-1);

int sig(double r) { return (r < -EPS) ? -1 : (r > +EPS) ? +1 : 0; }

struct Pt{
	double x, y;
	Pt() {}
	Pt(double x, double y) : x(x), y(y) {}
	Pt operator+(const Pt &a) const { return Pt(x + a.x, y + a.y); }
	Pt operator-(const Pt &a) const { return Pt(x - a.x, y - a.y); }
	Pt operator*(const Pt &a) const { return Pt(x * a.x - y * a.y, x * a.y + y * a.x); }
	Pt operator-() const { return Pt(-x, -y); }
	Pt operator*(const double &k) const { return Pt(x * k, y * k); }
	Pt operator/(const double &k) const { return Pt(x / k, y / k); }
	double abs() const { return sqrt(x * x + y * y); }
	double abs2() const { return x * x + y * y; }
	double arg() const { return atan2(y, x); }
	double dot(const Pt &a) const { return x * a.x + y * a.y; }
	double det(const Pt &a) const { return x * a.y - y * a.x; }
};

ostream &operator<<(ostream &os, const Pt &a) { os << "(" << a.x << ", " << a.y << ")"; return os; }

double tri(const Pt &a, const Pt &b, const Pt &c) { return (b - a).det(c - a); }

int iSP(Pt a, Pt b, Pt c) {
	int s = sig((b - a).det(c - a));
	if (s) return s; // +1 or -1
	if (sig((b - a).dot(c - a)) < 0) return -2; // c-a-b
	if (sig((a - b).dot(c - b)) < 0) return +2; // a-b-c
	return 0;
}
int iLL(Pt a, Pt b, Pt c, Pt d) {
	if (sig((b - a).det(d - c))) return 1; // intersect
	if (sig((b - a).det(c - a))) return 0; // parallel
	return -1; // correspond
}
bool iLS(Pt a, Pt b, Pt c, Pt d) {
	return (sig(tri(a, b, c)) * sig(tri(a, b, d)) <= 0);
}
bool iSS(Pt a, Pt b, Pt c, Pt d) {
	return (iSP(a, b, c) * iSP(a, b, d) <= 0 && iSP(c, d, a) * iSP(c, d, b) <= 0);
}
bool iSSstrict(Pt a, Pt b, Pt c, Pt d) {
	return (sig(tri(a, b, c)) * sig(tri(a, b, d)) < 0 && sig(tri(c, d, a)) * sig(tri(c, d, b)) < 0);
}
Pt pLL(Pt a, Pt b, Pt c, Pt d) { // intersection
	b = b - a; d = d - c; return a + b * (c - a).det(d) / b.det(d);
}
double dLP(Pt a, Pt b, Pt c) {
	return abs(tri(a, b, c)) / (b - a).abs();
}
double dSP(Pt a, Pt b, Pt c) {
	if (sig((b - a).dot(c - a)) <= 0) return (c - a).abs();
	if (sig((a - b).dot(c - b)) <= 0) return (c - b).abs();
	return abs(tri(a, b, c)) / (b - a).abs();
}
double dLL(Pt a, Pt b, Pt c, Pt d) {
	return iLL(a, b, c, d) ? 0 : dLP(a, b, c);
}
double dLS(Pt a, Pt b, Pt c, Pt d) {
	return iLS(a, b, c, d) ? 0 : min(dLP(a, b, c), dLP(a, b, d));
}
double dSS(Pt a, Pt b, Pt c, Pt d) {
	return iSS(a, b, c, d) ? 0 : min(min(dSP(a, b, c), dSP(a, b, d)), min(dSP(c, d, a), dSP(c, d, b)));
}

int iCC(Pt a, double r, Pt b, double s) {
	double d = (b - a).abs();
	if (sig(d) == 0 && sig(r - s) == 0) return -1; // correspond
	if (sig(r - s - d) > 0) return +2; // r > s
	if (sig(s - r - d) > 0) return -2; // s > r
	return (sig(r + s - d) >= 0) ? 1 : 0;
}
bool iCS(Pt a, double r, Pt b, Pt c) {
	return (sig(r - dSP(b, c, a)) >= 0 && sig(r - max((b - a).abs(), (c - a).abs())) <= 0);
}
pair<Pt,Pt> pCC(Pt a, double r, Pt b, double s) {
	double d = (b - a).abs();
	double x = (d * d + r * r - s * s) / (d * 2);
	Pt e = (b - a) / d, w = e * Pt(0, 1) * sqrt(max(r * r - x * x, 0.0));
	return mp(a + e * x - w, a + e * x + w);
}
pair<Pt,Pt> pCL(Pt a, double r, Pt b, Pt c) {
	Pt h = b + (c - b) * (c - b).dot(a - b) / (c - b).abs2();
	// perp(b, c, a)
	double d = (h - a).abs();
	double y = sqrt(max(r * r - d * d, 0.0));
	Pt e = (c - b) / (c - b).abs();
	return mp(h - e * y, h + e * y);
}
pair<Pt,Pt> tCP(Pt a, double r, Pt b) {
	double d2 = (b - a).abs2();
	double x = sqrt(max(d2 - r * r, 0.0));
	Pt h = a + (b - a) * (r * r / d2);
	Pt w = (b - a) * Pt(0, 1) * (x * r / d2);
	return mp(h - w, h + w);
}
double aCC(Pt a, double r, Pt b, double s) {
	double d = (a - b).abs();
	if (sig(r - s - d) >= 0) return s * s * PI;
	if (sig(s - r - d) >= 0) return r * r * PI;
	if (sig(r + s - d) <= 0) return 0;
	double x = (d * d + r * r - s * s) / (d * 2);
	double h = sqrt(r * r - x * x);
	return r * r * atan2(h, x) + s * s * atan2(h, d - x) - d * h;
}

// end of library

int NA,NB;
Pt PA[1010],PB[1010];

typedef pair<double,int> pdi;

double solve(int N,Pt* P,Pt C,Pt D){
	priority_queue<pdi,vector<pdi>,greater<pdi>> Q;
	Q.push(mp(0,0));
	double d[1010];
	rep(i,N)d[i]=i==0?0:INF;
	while(sz(Q)){
		pdi p=Q.top();Q.pop();
		double dist=p.fir;
		int cur=p.sec;
		if(dist>d[cur])continue;
		if(cur==1)return d[1]+(D-C).abs();
		rep(i,N){
			if(!iSS(P[cur],P[i],C,D)){
				double ndist=dist+(P[i]-P[cur]).abs();
				if(ndist<d[i]){
					d[i]=ndist;
					Q.push(mp(ndist,i));
				}
			}
		}
	}
	return INF;
}

int main(){
	cin.tie(0);
	ios_base::sync_with_stdio(0);
	
	cin>>NA>>NB;
	rep(i,NA)cin>>PA[i].x>>PA[i].y;
	rep(i,NB)cin>>PB[i].x>>PB[i].y;
	double ans=min(solve(NA,PA,PB[0],PB[1]),solve(NB,PB,PA[0],PA[1]));
	cout<<setprecision(16)<<(ans==INF?-1:ans)<<endl;
}

Measured memory_bytes: 1472