#include<stdio.h>
#include <iostream>
#include <math.h>
#include <numeric>
#include <vector>
#include <map>
#include <functional>
#include <stdio.h>
#include <array>
#include <algorithm>
#include <string>
#include <assert.h>
#include <stdio.h>
#include <queue>
#include<iomanip>
#include<bitset>
#include<stack>
using namespace std;

const long long int mod = 1000000007;

struct Mod {
	int num;
	Mod() : num(0) { ; }
	Mod(long long int n) : num((n % mod + mod) % mod) { ; }
	Mod(int n) : num((n % mod + mod) % mod) { ; }
	operator int() { return num; }
};

Mod operator+(Mod a, Mod b) { return Mod((a.num + b.num) % mod); }
Mod operator+(long long int a, Mod b) { return Mod(a) + b; }
Mod operator-(Mod a, Mod b) { return Mod((mod + a.num - b.num) % mod); }
Mod operator-(long long int a, Mod b) { return Mod(a) - b; }
Mod operator*(Mod a, Mod b) { return Mod(((long long)a.num * b.num) % mod); }
Mod operator*(long long int a, Mod b) { return Mod(a)*b; }
Mod operator+=(Mod &a, Mod b) { return a = a + b; }
Mod operator+=(long long int &a, Mod b) { return a = a + b; }
Mod operator-=(Mod &a, Mod b) { return a = a - b; }
Mod operator-=(long long int &a, Mod b) { return a = a - b; }
Mod operator*=(Mod &a, Mod b) { return a = a * b; }
Mod operator*=(long long int &a, Mod b) { return a = a * b; }
Mod operator^(Mod a, long long int n) {
	if (n == 0) return Mod(1);
	Mod res = (a * a) ^ (n / 2);
	if (n % 2) res = res * a;
	return res;
}
Mod inv(Mod a) { return a ^ (mod - 2); }
Mod operator/(Mod a, Mod b) {
	assert(b != 0);
	return a * inv(b);
}
Mod operator/(long long int a, Mod b) {
	assert(b != 0);
	return Mod(a) * inv(b);
}

#define MAX_N 1024000

Mod fact[MAX_N], factinv[MAX_N];
void init() {
	fact[0] = Mod(1); factinv[0] = 1;
	for (int i = 0; i < MAX_N - 1; ++i) {
		fact[i + 1] = fact[i] * Mod(i + 1);
		factinv[i + 1] = factinv[i] / Mod(i + 1);
	}
}
Mod comb(int a, int b) {
	return fact[a] * factinv[b] * factinv[a - b];
}


#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(x) (x).begin(),(x).end()



/* 幾何の基本 */

#include <complex>

typedef long double ld;
typedef complex<ld> Point;

const ld eps = 1e-9, pi = acos(-1.0);

namespace std {
  bool operator<(const Point &lhs, const Point &rhs) {
    if (lhs.real() < rhs.real() - eps) return true;
    if (lhs.real() > rhs.real() + eps) return false;
    return lhs.imag() < rhs.imag();
  }
}

// 点の入力
Point input_point() {
  ld x, y;
  cin >> x >> y;
  return Point(x, y);
}

// 誤差つき等号判定
bool eq(ld a, ld b) {
  return (abs(a - b) < eps);
}

// 内積
ld dot(Point a, Point b) {
  return real(conj(a) * b);
}

// 外積
ld cross(Point a, Point b) {
  return imag(conj(a) * b);
}

// 直線の定義
class Line {
public:
  Point a, b;
  Line () : a(Point(0, 0)), b(Point(0, 0)) {}
  Line (Point a, Point b) : a(a), b(b) {}
};

// 円の定義
class Circle {
public:
  Point p;
  ld r;
  Circle () : p(Point(0, 0)), r(0) {}
  Circle (Point p, ld r) : p(p), r(r) {}
};

// CCW
int ccw (Point a, Point b, Point c) {
  b -= a; c -= a;
  if (cross(b, c) > eps) return 1;   // a,b,cが反時計周りの順に並ぶ
  if (cross(b, c) < -eps) return -1; // a,b,cが時計周りの順に並ぶ
  if (dot(b, c) < 0) return 2;       // c,a,bの順に直線に並ぶ
  if (norm(b) < norm(c)) return -2;  // a,b,cの順に直線に並ぶ
  return 0;                          // a,c,bの順に直線に並ぶ
}


/* 交差判定 */

// 直線と直線の交差判定
bool isis_ll (Line l, Line m) {
  return !eq(cross(l.b - l.a, m.b - m.a), 0);
}

// 直線と線分の交差判定
bool isis_ls (Line l, Line s) {
  return isis_ll(l, s) &&
    (cross(l.b - l.a, s.a - l.a) * cross(l.b - l.a, s.b - l.a) < eps);
}

// 線分と線分の交差判定
bool isis_ss(Line s, Line t) {
  return ccw(s.a, s.b, t.a) * ccw(s.a, s.b, t.b) <= 0 &&
    ccw(t.a, t.b, s.a) * ccw(t.a, t.b, s.b) <= 0;
}

// 点の直線上判定
bool isis_lp (Line l, Point p) {
  return (abs(cross(l.b - p, l.a - p)) < eps);
}

// 点の線分上判定
bool isis_sp (Line s, Point p) {
  return (abs(s.a - p) + abs(s.b - p) - abs(s.b - s.a) < eps);
}

// 垂線の足
Point proj (Line l, Point p) {
  ld t = dot(p - l.a, l.a - l.b) / norm(l.a - l.b);
  return l.a + t * (l.a - l.b);
}

// 直線と直線の交点
Point is_ll (Line s, Line t) {
  Point sv = s.b - s.a, tv = t.b - t.a;
  assert(cross(sv, tv) != 0);
  return s.a + sv * cross(tv, t.a - s.a) / cross(tv, sv);
}

// 直線と点の距離
ld dist_lp (Line l, Point p) {
  return abs(p - proj(l, p));
}

// 直線と直線の距離
ld dist_ll (Line l, Line m) {
  return isis_ll(l, m) ? 0 : dist_lp(l, m.a);
}

// 直線と線分の距離
ld dist_ls (Line l, Line s) {
  return isis_ls(l, s) ? 0 : min(dist_lp(l, s.a), dist_lp(l, s.b));
}

// 線分と点の距離
ld dist_sp (Line s, Point p) {
  Point r = proj(s, p);
  return isis_sp(s, r) ? abs(r - p) : min(abs(s.a - p), abs(s.b - p));
}

// 線分と線分の距離
ld dist_ss (Line s, Line t) {
  if (isis_ss(s, t)) return 0;
  return min({dist_sp(s, t.a), dist_sp(s, t.b), dist_sp(t, s.a), dist_sp(t, s.b)});
}


/* 円 */

// 円と円の交点
vector<Point> is_cc (Circle c1, Circle c2){
  vector<Point> res;
  ld d = abs(c1.p - c2.p);
  ld rc = (d * d + c1.r * c1.r - c2.r * c2.r) / (2 * d);
  ld dfr = c1.r * c1.r - rc * rc;
  if (abs(dfr) < eps) dfr = 0.0;
  else if (dfr < 0.0) return res; // no intersection
  ld rs = sqrt(dfr);
  Point diff = (c2.p - c1.p) / d;
  res.push_back(c1.p + diff * Point(rc, rs));
  if (dfr != 0.0) res.push_back(c1.p + diff * Point(rc, -rs));
  return res;
}

// 円と直線の交点
vector<Point> is_lc (Circle c, Line l){
  vector<Point> res;
  ld d = dist_lp(l, c.p);
  if (d < c.r + eps){
    ld len = (d > c.r) ? 0.0 : sqrt(c.r * c.r - d * d); //safety;
    Point nor = (l.a - l.b) / abs(l.a - l.b);
    res.push_back(proj(l, c.p) + len * nor);
    res.push_back(proj(l, c.p) - len * nor);
  }
  return res;
}

// 円と線分の距離
vector<Point> is_sc(Circle c, Line l){
  vector<Point> v = is_lc(c, l), res;
  for (Point p : v)
    if (isis_sp(l, p)) res.push_back(p);
  return res;
}

// 円と点の接線
vector<Line> tangent_cp(Circle c, Point p) {
  vector<Line> ret;
  Point v = c.p - p;
  ld d = abs(v);
  ld l = sqrt(norm(v) - c.r * c.r);
  if (isnan(l)) { return ret; }
  Point v1 = v * Point(l / d,  c.r / d);
  Point v2 = v * Point(l / d, -c.r / d);
  ret.push_back(Line(p, p + v1));
  if (l < eps) return ret;
  ret.push_back(Line(p, p + v2));
  return ret;
}

// 円と円の接線
vector<Line> tangent_cc(Circle c1, Circle c2) {
  vector<Line> ret;
  if (abs(c1.p - c2.p) - (c1.r + c2.r) > -eps) {
    Point center = (c1.p * c2.r + c2.p * c1.r) / (c1.r + c2.r);
    ret = tangent_cp(c1, center);
  }
  if (abs(c1.r - c2.r) > eps) {
    Point out = (-c1.p * c2.r + c2.p * c1.r) / (c1.r - c2.r);
    vector<Line> nret = tangent_cp(c1, out);
    ret.insert(ret.end(), ALL(nret));
  }
  else {
    Point v = c2.p - c1.p;
    v /= abs(v);
    Point q1 = c1.p + v * Point(0,  1) * c1.r;
    Point q2 = c1.p + v * Point(0, -1) * c1.r;
    ret.push_back(Line(q1, q1 + v));
    ret.push_back(Line(q2, q2 + v));
  }
  return ret;
}


/* 多角形 */

typedef vector<Point> Polygon;

// 面積
ld area(const Polygon &p) {
  ld res = 0;
  int n = p.size();
  REP(j,n) res += cross(p[j], p[(j+1)%n]);
  return res / 2;
}

// 多角形の回転方向
bool is_counter_clockwise (const Polygon &poly) {
  ld angle = 0;
  int n = poly.size();
  REP(i,n) {
    Point a = poly[i], b = poly[(i+1)%n], c = poly[(i+2)%n];
    angle += arg((c - b) / (b - a));
  }
  return angle > eps;
}

// 円の内外判定
// -1 => out
//  0 => on
//  1 => in
int is_in_polygon (const Polygon &poly, Point p) {
  ld angle = 0;
  int n = poly.size();
  REP(i,n) {
    Point a = poly[i], b = poly[(i+1)%n];
    if (isis_sp(Line(a, b), p)) return 1;
    angle += arg((b - p) / (a - p));
  }
  return eq(angle, 0) ? 0 : 2;
}

// 凸包
Polygon convex_hull(vector<Point> ps) {
  int n = ps.size();
  int k = 0;
  sort(ps.begin(), ps.end());
  Polygon ch(2 * n);
  for (int i = 0; i < n; ch[k++] = ps[i++])
    while (k >= 2 && ccw(ch[k - 2], ch[k - 1], ps[i]) <= 0) --k;
  for (int i = n - 2, t = k + 1; i >= 0; ch[k++] = ps[i--])
    while (k >= t && ccw(ch[k - 2], ch[k - 1], ps[i]) <= 0) --k;
  ch.resize(k - 1);
  return ch;
}

// 凸カット
Polygon convex_cut(const Polygon &ps, Line l) {
  int n = ps.size();
  Polygon Q;
  REP(i,n) {
    Point A = ps[i], B = ps[(i+1)%n];
    Line m = Line(A, B);
    if (ccw(l.a, l.b, A) != -1) Q.push_back(A);
    if (ccw(l.a, l.b, A) * ccw(l.a, l.b, B) < 0 && isis_ll(l, m))
      Q.push_back(is_ll(l, m));
  }
  return Q;
}


/* アレンジメント */
void add_point(vector<Point> &ps, Point p) {
  for (Point q : ps) if (abs(q - p) < eps) return;
  ps.push_back(p);
}

typedef int Weight;

struct Edge { int from, to; Weight weight; };

typedef vector<Edge> Edges;
typedef vector<Edges> Graph;

void add_edge(Graph &g, int from, int to, Weight weight) {
  g[from].push_back(Edge{from, to, weight});
}

Graph segment_arrangement(const vector<Line> &s, const vector<Point> &p) {
  int n = p.size(), m = s.size();
  Graph g(n);
  REP(i,m) {
    vector<pair<ld,int>> vec;
    REP(j,n) if (isis_sp(s[i], p[j]))
      vec.emplace_back(abs(s[i].a - p[j]), j);
    sort(ALL(vec));
    REP(j,vec.size()-1) {
      int from = vec[j].second, to = vec[j+1].second;
      add_edge(g, from, to, abs(p[from] - p[to]));
    }
  }
  return g;
}

Graph circle_arrangement(const vector<Circle> &c, const vector<Point> &p) {
  int n = p.size(), m = c.size();
  Graph g(n);
  REP(i,m) {
    vector<pair<ld,int>> vec;
    REP(j,n) if (abs(abs(c[i].p - p[j]) - c[i].r) < eps)
      vec.emplace_back(arg(c[i].p - p[j]), j);
    sort(ALL(vec));
    REP(j,vec.size()-1) {
      int from = vec[j].second, to = vec[j+1].second;
      ld angle = vec[j+1].first - vec[j].first;
      add_edge(g, from, to, angle * c[i].r);
    }
    if (vec.size() >= 2) {
      int from = vec.back().second, to = vec.front().first;
      ld angle = vec.front().first - vec.back().first;
      add_edge(g, from, to, angle * c[i].r);
    }
  }
  return g;
}


/* 双対グラフ */

// 線分集合は既にアレンジメントされていなければならない．
// 内側の円は時計回りで，外側の円は反時計回りで得られる．
// 変数 polygon は，vector<int> で表される多角形の集合であり，
// vector<int> で表される 多角形のi番目は，その頂点の頂点集合pにおける番号である．
vector<vector<int>> polygon;
vector<int> seg2p[1024][1024];

Graph dual_graph(const vector<Line> &s, const vector<Point> &p) {
  int N = p.size();
  polygon.clear();
  REP(i,1024) REP(j,1024) seg2p[i][j].clear();
  vector<vector<tuple<ld,int,bool>>> tup(N);
  REP(i,s.size()) {
    int a = -1, b = -1;
    REP(j,N) if (abs(s[i].a - p[j]) < eps) a = j;
    REP(j,N) if (abs(s[i].b - p[j]) < eps) b = j;
    assert(a >= 0 && b >= 0);
    tup[a].emplace_back(arg(s[i].b - s[i].a), b, false);
    tup[b].emplace_back(arg(s[i].a - s[i].b), a, false);
  }
  REP(i,N) sort(ALL(tup[i]));
  REP(i,N) {
    REP(j,tup[i].size()) {
      ld angle; int pos = j, from = i, to; bool flag;
      tie(angle, to, flag) = tup[i][j];
      if (flag) continue;
      vector<int> ps;
      while (!flag) {
        ps.push_back(from);
        get<2>(tup[from][pos]) = true;
        seg2p[from][to].push_back(polygon.size());
        seg2p[to][from].push_back(polygon.size());
        angle += pi + eps;
        if (angle > pi) angle -= 2 * pi;
        auto it = lower_bound(ALL(tup[to]), make_tuple(angle, 0, false));
        if (it == tup[to].end()) it = tup[to].begin();
        from = to; tie(angle, to, flag) = *it;
        pos = it - tup[from].begin();
      }
      polygon.push_back(ps);
    }
  }
  Graph g(polygon.size());
  REP(i,N) REP(j,i) {
    if (seg2p[i][j].size() == 2) {
      int from = seg2p[i][j][0], to = seg2p[i][j][1];
      g[from].push_back(Edge{from, to});
      g[to].push_back(Edge{to, from});
    }
  }
  return g;
}


/* ビジュアライザ */
const ld zoom = 25;
const ld centerX = 6;
const ld centerY = 5;

void change_color(int r, int g, int b) {
  fprintf(stderr, "c.strokeStyle = 'rgb(%d, %d, %d)';\n", r, g, b);
}

int cordx(Point p) { return 400 + zoom * (p.real() - centerX); }
int cordy(Point p) { return 400 - zoom * (p.imag() - centerY); }

#define cord(p) cordx(p),cordy(p)

void draw_point(Point p) {
  fprintf(stderr, "circle(%d, %d, %d)\n", cord(p), 2);
}

void draw_segment(Line l) {
  fprintf(stderr, "line(%d, %d, %d, %d)\n", cord(l.a), cord(l.b));
}

void draw_line(Line l) {
  Point v = l.b - l.a;
  Line m(l.a - v * Point(1e4, 0), l.b + v * Point(1e4, 0));
  fprintf(stderr, "line(%d, %d, %d, %d)\n", cord(m.a), cord(m.b));
}

void draw_polygon(const Polygon &p) {
  int n = p.size();
  REP(i,n) draw_segment(Line(p[i], p[(i+1)%n]));
}

void draw_circle(Circle c) {
  fprintf(stderr, "circle(%d, %d, %d)\n", cord(c.p), (int)(zoom * c.r));
}

class Compare {
public:
	bool operator()(const pair<int, double>&l, const pair<int, double>&r) {
		return l.second > r.second;
	}
};

int main() {
	int a, b;
	cin >> a >> b;
	vector<Point> totaps;
	vector<Point> tuteps;
	for (int i = 0; i < a; ++i) {
		double x;
		double y;
		cin >> x >> y;
		totaps.push_back(Point(x, y));
	}
	for (int i = 0; i < b; ++i) {
		double x;
		double y;
		cin >> x >> y;
		tuteps.push_back(Point(x, y));
	}
	double ans1 = 0;
	{
		Line totaload(totaps[0], totaps[1]);
		vector<vector<pair<int,double>>>Nodes(b);
		for (int i = 0; i < b; ++i) {
			for (int j = 0; j < b; ++j) {
				if (i != j) {
					if (!isis_ss(totaload, Line(tuteps[i], tuteps[j]))) {
						double dis = abs(tuteps[i] - tuteps[j]);
						Nodes[i].push_back(make_pair(j, dis));
					}
				}
			}
		}
		vector<double>times(b, 999999999999);
		times[0] = 0;
		priority_queue<pair<int, double>,vector<pair<int, double>>,Compare>qu;
		qu.push(make_pair(0, 0));
		while (!qu.empty()) {
			pair<int, double> atop = qu.top();
			qu.pop();
			for (int i = 0; i < Nodes[atop.first].size(); ++i) {
				if (times[Nodes[atop.first][i].first]>atop.second + Nodes[atop.first][i].second) {
					qu.push(make_pair(Nodes[atop.first][i].first,atop.second + Nodes[atop.first][i].second));
					times[Nodes[atop.first][i].first] = atop.second + Nodes[atop.first][i].second;
				}
			}
		}
		ans1= abs(totaload.a-totaload.b);
		ans1 += times[1];
	}
	double ans2 = 0;
	{
		Line tuteload(tuteps[0], tuteps[1]);
		vector<vector<pair<int, double>>>Nodes(a);
		for (int i = 0; i < a; ++i) {
			for (int j = 0; j < a; ++j) {
				if (i != j) {
					if (!isis_ss(tuteload, Line(totaps[i], totaps[j]))) {
						double dis=abs(totaps[i] - totaps[j]);
						Nodes[i].push_back(make_pair(j, dis));
					}
				}
			}
		}
		vector<double>times(a, 999999999999);
		times[0] = 0;
		priority_queue<pair<int, double>, vector<pair<int, double>>, Compare>qu;
		qu.push(make_pair(0, 0));
		while (!qu.empty()) {
			pair<int, double> atop = qu.top();
			qu.pop();
			for (int i = 0; i < Nodes[atop.first].size(); ++i) {
				if (times[Nodes[atop.first][i].first]>atop.second + Nodes[atop.first][i].second) {
					qu.push(make_pair(Nodes[atop.first][i].first, atop.second + Nodes[atop.first][i].second));
					times[Nodes[atop.first][i].first] = atop.second + Nodes[atop.first][i].second;
				}
			}
		}
		ans2 = abs(tuteload.a - tuteload.b);
		ans2 += times[1];
	}
	double ans = min(ans1, ans2);
	if (ans > 999999999990) {
		cout << -1 << endl;
	}else{
		cout <<fixed<<setprecision(20)<< ans << endl;
	}
	return 0;	
}

Measured memory_bytes: 51820