////////////////////////////////////////
///  tu3 pro-con template            ///
////////////////////////////////////////
#include <cassert>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <numeric>
#include <functional>
#include <vector>
#include <queue>
#include <string>
#include <complex>
#include <stack>
#include <set>
#include <map>
#include <list>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <regex>
using namespace std;

//// MACRO ////
#define countof(a) (sizeof(a)/sizeof(a[0]))

#define REP(i,n) for (int i = 0; i < (n); i++)
#define RREP(i,n) for (int i = (n)-1; i >= 0; i--)
#define FOR(i,s,n) for (int i = (s); i < (n); i++)
#define RFOR(i,s,n) for (int i = (n)-1; i >= (s); i--)
#define pos(c,i) c.being() + (i)
#define allof(c) c.begin(), c.end()
#define aallof(a) a, countof(a)
#define partof(c,i,n) c.begin() + (i), c.begin() + (i) + (n)
#define apartof(a,i,n) a + (i), a + (i) + (n)
#define long long long

#define EPS 1e-9
#define INF (1L << 30)
#define LINF (1LL << 60)

#define PREDICATE(t,a) (const t & a) -> bool
#define COMPARISON_T(t) bool(*)(const t &, const t &)
#define COMPARISON(t,a,b) (const t & a, const t & b) -> bool
#define CONVERTER(TSrc,t,TDest) (const TSrc &t)->TDest

//// i/o helper ////

struct _Reader { template <class T> _Reader operator ,(T &rhs) { cin >> rhs; return *this; } };
struct _Writer { bool f; _Writer() : f(false) { } template <class T> _Writer operator ,(const T &rhs) { cout << (f ? " " : "") << rhs; f = true; return *this; } };
#define READ(t,...) t __VA_ARGS__; _Reader(), __VA_ARGS__
#define WRITE(...) _Writer(), __VA_ARGS__; cout << endl
inline int sign(double x) { return (abs(x) < EPS ? 0 : x > 0 ? 1 : -1); }

template <class T> struct vevector : public vector<vector<T>> { vevector(int n = 0, int m = 0, const T &initial = T()) : vector<vector<T>>(n, vector<T>(m, initial)) { } };
template <class T> struct vevevector : public vector<vevector<T>> { vevevector(int n = 0, int m = 0, int l = 0, const T &initial = T()) : vector<vevector<T>>(n, vevector<T>(m, l, initial)) { } };
template <class T> struct vevevevector : public vector<vevevector<T>> { vevevevector(int n = 0, int m = 0, int l = 0, int k = 0, const T &initial = T()) : vector<vevevector<T>>(n, vevevector<T>(m, l, k, initial)) { } };
template <class T> T read() { T t; cin >> t; return t; }
template <class T> vector<T> read(int n) { vector<T> v; REP(i, n) { v.push_back(read<T>()); } return v; }
template <class T> vevector<T> read(int n, int m) { vevector<T> v; REP(i, n) v.push_back(read<T>(m)); return v; }
template <class T> vevector<T> readjag() { return read<T>(read<int>()); }
template <class T> vevector<T> readjag(int n) { vevector<T> v; REP(i, n) v.push_back(readjag<T>()); return v; }

template <class T1, class T2> inline istream & operator >> (istream & in, pair<T1, T2> &p) { in >> p.first >> p.second; return in; }
template <class T1, class T2> inline ostream & operator << (ostream &out, pair<T1, T2> &p) { out << p.first << p.second; return out; }
template <class T> inline ostream & operator << (ostream &out, vector<T> &v)
{
	ostringstream ss;
	for (auto x : v) ss << x << ' ';
	auto s = ss.str();
	out << s.substr(0, s.length() - 1) << endl;
	return out;
}

//// start up ////
void solve();
int main()
{
	solve();
	return 0;
}

//// bit ////
#ifdef _MSC_VER
#pragma push_macro("long")
#undef long
inline unsigned __builtin_ctz(unsigned x) { unsigned long r; _BitScanReverse(&r, x); return r; }
inline unsigned __builtin_clz(unsigned x) { unsigned long r; _BitScanForward(&r, x); return r; }
inline unsigned msb(unsigned v) { v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16; return (v >> 1) + 1; }
inline unsigned round_up_pow2(unsigned v) { return msb(v - 1) << 1; }
inline unsigned msb_index(unsigned v) { static const int pos[32] = { 0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9 }; return pos[(v * 0x077CB531UL) >> 27]; }
#pragma pop_macro("long")
#endif

inline int next_bit_permutation(int x)
{
	int t = x | (x - 1);
	return (t + 1) | (unsigned)((~t & -~t) - 1) >> (__builtin_ctz(x) + 1);
}

// x の p1からlen bitと p2 から len bit を入れ替える
template<class T>
int bit_swap(T x, size_t p1, size_t p2, size_t len)
{
	T mask = (1ul << len) - 1;
	T ope = (x >> p1 ^ x >> p2) & mask; // 入れ替える場所のxorを求める
	return x ^ (ope << p1 | ope << p2); // 入れ替える場所をxor。
}

//// algorithm ////

// compare
template <class InIt1, class InIt2>
int partial_compare(InIt1 first1, InIt1 last1, InIt2 first2, InIt2 last2)
{
	return lexicographical_compare(first1, last1, first2, last2) ? -1
		: lexicographical_compare(first2, last2, first1, last1) ? 1
		: 0;
}

// index_of
template <class InIt, class T>
inline int index_of(InIt first1, InIt last1, const T &val)
{
	InIt f = find(first1, last1, val);
	return f != last1 ? distance(f, first1) : -1;
}

// C# BinarySearch風のindex_of
template <class InIt, class T>
inline int bindex_of(InIt first1, InIt last1, const T &val)
{
	auto it = lower_bound(first1, last1, val);
	if (it != last1 && *it == val) { return distance(it, first1); }
	return ~distance(it, first1);
}

// iota vector 
vector<int> iotav(int begin, int end)
{
	vector<int> r(end - begin);
	iota(allof(r), begin);
	return r;
}

// iota イテレータ
struct iotait
{
	int n;
	iotait(int n = 0) : n(n) { }
	iotait &operator ++() { ++n; return *this; }
	int operator *() { return n; }
};

// 文字列の置換
void inplaceReplace(string &target, const string &from, const string &to, bool global = false)
{
	do
	{
		string::size_type pos = target.find(from);
		if (pos == target.npos) { break; }
		target.replace(pos, from.length(), to);
	} while (true);
}

// 範囲内チェック
inline bool contains(int x, int min, int max) { return x >= min && x < max; }
inline bool contains(int x, int y, int W, int H) { return x >= 0 && x < W && y >= 0 && y < H; }

//// data structure ////

/// 2分セグメント木。RMQとかが解ける。
template<class T = int, class TParentFunc = const T& (*)(const T&, const T&)>
struct SegTree
{
	int size;
	vector<T> tree;
	TParentFunc parent;
	unsigned msb(unsigned v) { REP(i, 5) v |= v >> (1 << i); return (v >> 1) + 1; }

	SegTree(int count, TParentFunc parentBuilder = std::min, T initialFill = INF)
		: size(msb(count - 1) << 1)
		, tree(size * 2)
		, parent(parentBuilder)
	{
		fill(partof(tree, size, size), initialFill);
		rebuild();
	}

	// []を使うと更新されないのでrebuildすること。数が少なければupdateの方がよい。
	T& operator[](int i) { return tree[i + size]; }
	void rebuild() { RREP(i, size) { tree[i] = parent(tree[i * 2], tree[i * 2 + 1]); } }

	void update(int index, T value)
	{
		tree[index + size] = value;
		for (int i = (index + size) / 2; i > 0; i /= 2)
			tree[i] = parent(tree[i * 2], tree[i * 2 + 1]);
	}

	T query(int s, int e, int i, int l, int r, T value)
	{
		if (r <= s || e <= l) return value;
		if (s <= l && r <= e) return parent(value, tree[i]);
		value = query(s, e, i * 2, l, (l + r) / 2, value);
		value = query(s, e, i * 2 + 1, (l + r) / 2, r, value);
		return value;
	}

	T query(int start, int count, T initialValue)
	{
		return query(start, start + count, 1, 0, size, initialValue);
	}
};


//// number ////

// 最大公約数
long gcd(long a, long b) { return b ? gcd(b, a%b) : a; }

// 最小公倍数
long lcm(long a, long b) { return a / gcd(a, b) * b; }

// 拡張ユークリッド互除法
void exgcd(long a, long b, long& x, long& y)
{
	if (b != 0) { exgcd(b, a%b, y, x); y -= a / b*x; }
	else { x = 1; y = 0; }
}

// mを法としたaの逆元を求める。mが素数なら、a**(m-2)がaの逆元。
long modInv(long a, long mod) { long x, y; exgcd(a, mod, x, y); return x; }

// modInvのvector版
vector<int> modInvVector(int n, int mod)
{
	vector<int> ret(n + 1);
	ret[1] = 1;
	for (int i = 2; i <= n; ++i)
		ret[i] = ret[mod % i] * (mod - mod / i) % mod;
	return ret;
}

// エラトステネス
vector<unsigned char> isPrime;
vector<int> primes;
void initPrimes(int n)
{
	isPrime = vector<unsigned char>(n + 1, true);
	isPrime[0] = isPrime[1] = false;
	FOR(i, 2, n + 1)
	{
		if (!isPrime[i]) continue;
		primes.push_back(i);
		for (int j = i * 2; j <= n; j += i)
			isPrime[j] = false;
	}
}

// パスカルの三角形(二項定理) 2種類の並べ替えにつかう。
vector<vector<double>> makePascalTriangle(int n, bool probability = false)
{
	typedef vector<double> VD;
	vector<VD> t;
	if (!t.size()) { t.push_back(VD(1, 1)); }
	FOR(i, t.size(), n + 1)
	{
		t.push_back(VD(i + 1));
		REP(j, i)
		{
			double x = t[i - 1][j] * (probability ? 0.5 : 1);
			t[i][j] += x;
			t[i][j + 1] += x;
		}
	}
	return t;
}

//// geometry ////
/// 3次元
struct P3
{
	double x, y, z;
	P3(double x = 0, double y = 0, double z = 0) : x(x), y(y), z(z) { }
	P3 operator +() const { return *this; }
	P3 operator +(const P3 &_) const { return P3(x + _.x, y + _.y, z + _.z); }
	P3 operator -() const { return P3(-x, -y, -z); }
	P3 operator -(const P3 &_) const { return *this + -_; }
	P3 operator *(double _) const { return P3(x*_, y*_, z*_); }
	P3 operator /(double _) const { return P3(x / _, y / _, z / _); }
	double dot(const P3 &_) const { return x*_.x + y*_.y + z*_.z; } // 内積
	P3 cross(const P3 &_) const { return P3(y*_.z - z*_.y, z*_.x - x*_.z, x*_.y - y*_.x); } // 外積
	double sqlength() const { return x*x + y*y + z*z; } // 二乗長さ
	double length() const { return sqrt(sqlength()); } // 長さ
	P3 direction() const { return *this / length(); } // 方向ベクトル
};
inline istream & operator>>(istream & in, P3 & p) { in >> p.x >> p.y >> p.z; return in; }
inline double abs(P3 p) { return p.length(); }

struct Sphere
{
	P3 c;
	double r;
	Sphere(double x, double y, double z, double r) : c(x, y, z), r(r) { }
	Sphere(P3 c, double r) : c(c), r(r) { }
	bool IntersectWith(const Sphere &rhs) const { return abs(c - rhs.c) - (r + rhs.r) < EPS; } // 接してても真。
};
inline istream & operator>>(istream & in, Sphere & c) { in >> c.c >> c.r; return in; }

/// 2次元
struct P2
{
	double x, y;
	P2(double x = 0, double y = 0) : x(x), y(y) { }
	P2(complex<double> c) : x(c.real()), y(c.imag()) { }
	P2 operator +() const { return *this; }
	P2 operator +(const P2 &_) const { return P2(x + _.x, y + _.y); }
	P2 operator -() const { return P2(-x, -y); }
	P2 operator -(const P2 &_) const { return *this + -_; }
	P2 operator *(double _) const { return P2(x*_, y*_); }
	P2 operator /(double _) const { return P2(x / _, y / _); }
	double dot(const P2 &_) const { return x*_.x + y*_.y; } // 内積
	double cross(const P2 &_) const { return x*_.y - y*_.x; } // 外積
	double sqlength() const { return x*x + y*y; } // 二乗長さ
	double length() const { return sqrt(sqlength()); } // 長さ
	P2 orthogonal() const { return P2(y, -x); }
	P2 direction() const { return *this / length(); } // 方向ベクトル
};
inline istream & operator>>(istream & in, P2 & p) { in >> p.x >> p.y; return in; }
inline ostream & operator<<(ostream & out, const P2 & p) { out << p.x << ' ' << p.y; return out; }
inline double abs(P2 p2) { return p2.length(); }
inline P2 orthogonal(P2 p) { return p.orthogonal(); }
inline complex<double> orthogonal(complex<double> c) { return  c * complex<double>(0, 1); }

// a,b から ちょうど d だけ離れた点。aとbを円周に持つ円の半径。
inline pair<P2, P2> get_same_distance_points(P2 a, P2 b, double d)
{
	assert(abs(a - b) <= 2 * d + EPS);
	auto v = (a + b) / 2.0 - a; // a から aとbの中点
	auto vl = abs(v);
	auto wl = sqrt(d*d - vl*vl); // 直行Vの大きさ
	auto w = orthogonal(v) * (wl / vl); // 直行V
	return make_pair(a + v + w, a + v - w);
}


struct Circle
{
	P2 c;
	double r;
	Circle() : c(), r() { }
	Circle(double x, double y, double r) : c(x, y), r(r) { }
	Circle(P2 c, double r) : c(c), r(r) { }
	bool IntersectWith(const Circle &rhs) const { return abs(c - rhs.c) - (r + rhs.r) < EPS; } // 接してても真。
	bool Contains(const P2 &p) const { return abs(p - c) - r < EPS; } // 接してても真。
};
inline istream & operator>>(istream & in, Circle & c) { in >> c.c >> c.r; return in; }

// 交差している2円の交点を求める。角度の小さい方から出る(右下方向が正なら時計回り)
inline pair<P2, P2> crossPoint(Circle A, Circle B)
{
	P2 v = B.c - A.c;
	P2 dir = v.direction(); // 他方の中心への方向
	double d = v.length(); // 他方の中心への距離
	double lh = (A.r*A.r + d*d - B.r*B.r) / (2 * d); // 垂線の足までの距離
	P2 h = A.c + dir * lh; // 垂線の足
	double lp = sqrt(A.r*A.r - lh*lh); // 垂線の足から交点までの距離
	P2 p = dir.orthogonal() * lp; // 垂線の足から交点へのベクトル
	return make_pair(h + p, h - p); // 交点の組。
}

inline int clockwise(P2 a, P2 b, P2 c)
{
	const P2 u = b - a, v = c - a;
	if (u.cross(v) > EPS) { return 1; }
	if (u.cross(v) < -EPS) { return -1; }
	if (u.dot(v) < -EPS) { return -1; }
	if (abs(u) - abs(v) < EPS) { return 1; }
	return 0;
}

struct Line
{
	P2 p, d;
	explicit Line(P2 pos = P2(), P2 dir = P2()) : p(pos), d(dir) { }
	static Line FromPD(P2 pos, P2 dir) { return Line(pos, dir); }
	static Line From2Point(P2 a, P2 b) { return Line(a, b - a); }
};

struct LineSeg
{
	P2 p, d;
	explicit LineSeg(P2 pos = P2(), P2 dir = P2()) : p(pos), d(dir) { }
	static LineSeg FromPD(P2 pos, P2 dir) { return LineSeg(pos, dir); }
	static LineSeg From2Point(P2 a, P2 b) { return LineSeg(a, b - a); }
};


inline P2 crossPoint(Line a, Line b) { return a.p + a.d * (b.d.cross(b.p - a.p) / b.d.cross(a.d)); }
inline bool contains(LineSeg a, P2 p) { return abs(a.p - p) + abs(a.p + a.d - p) - abs(a.d) < EPS; }

inline bool isCross(Line a, Line b) { return abs(a.d.cross(b.d)) > EPS; }
inline bool isCross(Line a, LineSeg b) { return sign(a.d.cross(b.p - a.p)) * sign(a.d.cross(b.p + b.d - a.d)) <= 0; }
inline bool isCross(LineSeg a, Line b) { return isCross(b, a); }
inline bool isCross(LineSeg a, LineSeg b)
{
	P2 ae = a.p + a.d, be = b.p + b.d;
	return clockwise(a.p, ae, b.p) * clockwise(a.p, ae, be) <= 0
		&& clockwise(b.p, be, a.p) * clockwise(b.p, be, ae) <= 0;
}

inline bool isOverlap(Line a, Line b) { return abs(a.d.cross(b.p - a.p)) < EPS; }
inline bool isOverlap(Line a, LineSeg b) { return isOverlap(a, Line(b.p, b.d)); }
inline bool isOverlap(LineSeg a, Line b) { return isOverlap(b, a); }
inline bool isOverlap(LineSeg a, LineSeg b)
{
	return contains(a, b.p) || contains(a, b.p + b.d)
		|| contains(b, a.p) || contains(b, a.p + a.d);
}


//// graph ////

template <class TCost = double>
struct Path
{
	typedef TCost COST;
	int from;
	int to;
	TCost cost;
	Path(int from = 0, int to = 0, TCost cost = 0)
		: from(from), to(to), cost(cost) { }
	bool operator < (const Path &rhs) const { return cost < rhs.cost; }
	bool operator >(const Path &rhs) const { return cost > rhs.cost; }
};

// prim //
template <class COST>
pair<COST, vector<int>> prim(const vector<vector<COST>> &costTable)
{
	typedef Path<COST> P;
	int N = costTable.size();
	priority_queue<P, vector<P>, greater<P>> q;
	q.push(P(0, 0, 0));
	vector<int> parent(N, -1);
	COST totalCost = 0;
	while (!q.empty())
	{
		P cur = q.top(); q.pop();
		int i = cur.to;
		if (parent[i] != -1) continue;
		parent[i] = cur.from;
		totalCost += cur.cost;
		REP(j, N) if (parent[j] == -1) q.push(P(i, j, costTable[i][j]));
	}
	return make_pair(totalCost, parent);
}

// dijkstra //
template <class COST>
pair<vector<COST>, vector<int>> dijkstra(const vector<vector<Path<COST>>> &routes, int start = 0, int goal = -1)
{
	typedef Path<COST> P;
	int N = routes.size();
	priority_queue<P, vector<P>, greater<P>> q;
	q.push(P(start, start, 0));

	vector<int> prev(N, -1);
	vector<COST> cost(N, INF);
	while (!q.empty())
	{
		P cur = q.top(); q.pop();
		int i = cur.to;
		if (prev[i] != -1) continue;
		prev[i] = cur.from;
		cost[i] = cur.cost;
		if (i == goal) { break; }
		REP(j, routes[i].size())
		{
			P next = P(i, routes[i][j].to, cur.cost + routes[i][j].cost);
			if (prev[i] != -1)
				q.push(next);
		}
	}
	return make_pair(cost, prev);
}

// warshall_floyd //
template <class COST>
void warshall_floyd(vector<vector<COST>> &cost)
{
	int n = cost.size();
	REP(k, n) REP(i, n) REP(j, n) cost[i][j] = min(cost[i][j], cost[i][k] + cost[k][j]);
}


//// dice ////

// さいころ
template<class T>
struct Die
{
	/* v as 天一地六東五西二南三北四
	|3|
	|1|0|4|5|
	|2|
	*/
	T v[6];

	void rt(int x, int y, int z, int w) { swap(v[x], v[y]); swap(v[x], v[z]); swap(v[x], v[w]); }
	void rotateLeft() { rt(0, 4, 5, 1); }
	void rotateRight() { rt(1, 5, 4, 0); }
	void rotateDown() { rt(0, 3, 5, 2); }
	void rotateUp() { rt(2, 5, 3, 0); }
	void rotateCw() { rt(3, 1, 2, 4); }
	void rotateCcw() { rt(4, 2, 1, 3); }
	bool operator < (const Die &rhs) const { return lexicographical_compare(aallof(v), aallof(rhs.v)); }
	bool operator == (const Die &rhs) const { return equal(aallof(v), rhs.v); }
	bool operator != (const Die &rhs) const { return !equal(aallof(v), rhs.v); }
};

// 8面体
template<class T>
struct Octahedra
{
	/* v as
	/0\/1\/2\/3\
	\4/\5/\6/\7/
	*/
	T v[8];

	void rt(int x, int y, int z, int w) { swap(v[x], v[y]); swap(v[x], v[z]); swap(v[x], v[w]); }
	void rotateLeft() { rt(0, 1, 2, 3); rt(4, 5, 6, 7); }
	void rotateRight() { rt(3, 2, 1, 0); rt(7, 6, 5, 4); }
	void rotateDown() { rt(0, 4, 7, 3); rt(1, 5, 6, 2); }
	void rotateUp() { rt(3, 7, 4, 0); rt(2, 6, 5, 2); }
	void rotateCw() { rt(0, 1, 5, 4); rt(3, 2, 6, 7); }
	void rotateCcw() { rt(4, 5, 1, 0); rt(7, 6, 1, 3); }
	bool operator < (const Octahedra &rhs) const { return lexicographical_compare(aallof(v), aallof(rhs.v)); }
	bool operator == (const Octahedra &rhs) const { return equal(aallof(v), rhs.v); }
	bool operator != (const Octahedra &rhs) const { return !equal(aallof(v), rhs.v); }
};


////////////////////
/// template end ///
////////////////////

void solve()
{
	int testcases = 1;
	REP(testcase, testcases)
	{
		READ(int, Na, Nb);
		auto Pa = read<P2>(Na);
		auto Pb = read<P2>(Nb);
		LineSeg ASeg = LineSeg::From2Point(Pa[0], Pa[1]);
		LineSeg BSeg = LineSeg::From2Point(Pb[0], Pb[1]);

		if (!isCross(ASeg, BSeg))
		{
			printf("%.15f", ASeg.d.length() + BSeg.d.length());
		}
		else
		{
			vevector<Path<double> > Ga(Na);
			vevector<Path<double> > Gb(Nb);
			
			REP(i, Na) REP(j, Na)
			{
				LineSeg s = LineSeg::From2Point(Pa[i], Pa[j]);
				if (!isCross(BSeg, s)) Ga[i].push_back({ i, j, s.d.length() });
			}
			
			REP(i, Nb) REP(j, Nb)
			{
				LineSeg s = LineSeg::From2Point(Pb[i], Pb[j]);
				if (!isCross(ASeg, s)) Gb[i].push_back({ i, j, s.d.length() });
			}

			double aconn = ASeg.d.length() + dijkstra(Gb, 0, 1).first[1];
			double bconn = BSeg.d.length() + dijkstra(Ga, 0, 1).first[1];
			double ans = min(aconn, bconn);
			if (ans > INF) { WRITE(-1); }
			else { printf("%.15f", ans); }
		}
	}
}

Measured memory_bytes: 47644