#include "bits/stdc++.h"
using namespace std;
// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=2334&lang=jp
typedef long long ll;
#define INF 1<<30
#define LINF 1LL<<60

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();
	}
}
// ????????\???
Point input_point() { ld x, y;cin >> x >> y;return Point(x, y); }

// ????????????????????????
bool eq(ld a, ld b) { return (abs(a - b) < eps); }

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

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

// ??´????????????
class Line {
public:
	Point a, b;
	Line() : a(Point(0, 0)), b(Point(0, 0)) {}
	Line(Point a, Point b) : a(a), b(b) {}
	Point operator[](const int _num) {
		if (_num == 0)return a;
		else if (_num == 1)return b;
		else assert(false);
	}
};

// ????????????
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??????????¨???¨?????????????????¶
	if (cross(b, c) < -eps) return -1; // a,b,c???????¨???¨?????????????????¶
	if (dot(b, c) < 0) return 2;       // c,a,b???????????´???????????¶
	if (norm(b) < norm(c)) return -2;  // a,b,c???????????´???????????¶
	return 0;                          // a,c,b???????????´???????????¶
}


/* ???????????? */

// ??´?????¨??´??????????????????
bool isis_ll(Line l, Line m) {
	return !eq(cross(l.b - l.a, m.b - m.a), 0);
}

// ??´?????¨?????????????????????
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);
}

// ????????¨?????????????????????
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;
}

// ????????´????????????
bool isis_lp(Line l, Point p) {
	return (abs(cross(l.b - p, l.a - p)) < eps);
}

// ?????????????????????
bool isis_sp(Line s, Point p) {
	return (abs(s.a - p) + abs(s.b - p) - abs(s.b - s.a) < eps);
}

// ??????????¶?
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);
}

// ??´?????¨??´????????????
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);
}

// ??´?????¨???????????¢
ld dist_lp(Line l, Point p) {
	return abs(p - proj(l, p));
}

// ??´?????¨??´???????????¢
ld dist_ll(Line l, Line m) {
	return isis_ll(l, m) ? 0 : dist_lp(l, m.a);
}

// ??´?????¨??????????????¢
ld dist_ls(Line l, Line s) {
	return isis_ls(l, s) ? 0 : min(dist_lp(l, s.a), dist_lp(l, s.b));
}

// ????????¨???????????¢
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));
}

// ????????¨??????????????¢
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) });
}

/* ??? */

// ?????¨????????????
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;
}

// ?????¨??´????????????
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;
}

// ?????¨??????????????¢
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;
}

// ?????¨????????\???
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;
}

// ?????¨????????\???
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(), nret.begin(), nret.end());
	}
	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;
}


/* ????§???¢ */

typedef vector<Point> Polygon;

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

// ????§???¢????????¢??????
bool is_counter_clockwise(const Polygon &poly) {
	ld angle = 0;
	int n = poly.size();
	for (int i = 0;i < n;i++) {
		Point a = poly[i], b = poly[(i + 1) % n], c = poly[(i + 2) % n];
		angle += arg((c - b) / (b - a));
	}
	return angle > eps;
}

// ??????????????????
// -1 => out
//  0 => on
//  1 => in
int is_in_polygon(const Polygon &poly, Point p) {
	ld angle = 0;
	int n = poly.size();
	for (int i = 0;i < n;i++) {
		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;
}

// ??????
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;
}

// ????????????
Polygon convex_cut(const Polygon &ps, Line l) {
	int n = ps.size();
	Polygon Q;
	for (int i = 0;i < n;i++) {
		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;
}

int main() {
	cin.tie(0); ios::sync_with_stdio(false);
	int NA, NB;cin >> NA >> NB;
	Polygon pa, pb;
	vector<vector<ld>> da(NA, vector<ld>(NA, INF)), db(NB, vector<ld>(NB, INF));
	for (int i = 0; i < NA;i++) {
		da[i][i] = 0;
		ld x, y;cin >> x >> y; pa.push_back(Point(x, y));
	}
	for (int i = 0; i < NB; i++) {
		db[i][i] = 0;
		ld x, y; cin >> x >> y; pb.push_back(Point(x, y));
	}

	for (int i = 0; i < NA;i++)for (int j = 0; j < NA;j++) {
		da[i][j] = da[j][i] = abs(pa[i] - pa[j]);
	}
	for (int i = 0; i < NB;i++)for (int j = 0; j < NB;j++) {
		db[i][j] = db[j][i] = abs(pb[i] - pb[j]);
	}

	ld ans = INF;
	ld sum = 0;
	/*    length of line segment A - B */
	sum += abs(pa[0] - pa[1]);
	Line la(pa[0], pa[1]);
	vector<vector<int>> fb(NB, vector<int>(NB, 0));
	for (int i = 0; i < NB;i++) {
		for (int j = i+1; j < NB;j++) {
			Line l(pb[i], pb[j]);
			if (isis_ss(la, l)) {
				fb[i][j] = fb[j][i] = 1;
			}
		}
	}
	queue<int> q; q.push(0);
	vector<ld> d(NB, INF);
	d[0] = 0;
	while (!q.empty()) {
		int idx = q.front(); q.pop();
		for (int i = 0; i < NB;i++) {
			if (i == idx)continue;
			if (fb[idx][i] == 1)continue;
			if (d[i] > d[idx] + db[idx][i]) {
				d[i] = d[idx] + db[idx][i];
				q.push(i);
			}
		}
	}
	if (d[1] != INF) { sum += d[1]; ans = min(ans, sum); }
	
	/*    length of line segment C - D*/
	sum = abs(pb[0] - pb[1]);
	Line lb(pb[0], pb[1]);
	vector<vector<int>> fa(NA, vector<int>(NA, 0));
	for (int i = 0; i < NA;i++) {
		for (int j = i + 1; j < NA;j++) {
			Line l(pa[i], pa[j]);
			if (isis_ss(lb, l)) {
				fa[i][j] = fa[j][i] = 1;
			}
		}
	}
	q.push(0);
	d.assign(NA, INF);
	d[0] = 0;
	while (!q.empty()) {
		int idx = q.front(); q.pop();
		for (int i = 0; i < NA;i++) {
			if (i == idx)continue;
			if (fa[idx][i] == 1)continue;
			if (d[i] > d[idx] + da[idx][i]) {
				d[i] = d[idx] + da[idx][i];
				q.push(i);
			}
		}
	}
	if (d[1] != INF) { sum += d[1]; ans = min(ans, sum); }
	if (ans == INF) {
		cout << -1 << endl;
	}
	else {
		cout << fixed << setprecision(10) << ans << endl;
	}
	return 0;
}