#include <iostream>
#include <sstream>
#include <string>
#include <algorithm>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <complex>
#include <cassert>

using namespace std;

#define FOR(i,k,n) for(int i=(k); i<(int)n; ++i)
#define REP(i,n) FOR(i,0,n)
#define FORIT(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)

const double EPS = 1e-8;
const double INF = 1e12;
typedef complex<double> P;
namespace std {
  bool operator < (const P& a, const P& b) {
    return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
  }
}
double cross(const P& a, const P& b) {
  return imag(conj(a)*b);
}
double dot(const P& a, const P& b) {
  return real(conj(a)*b);
}

struct L : public vector<P> {
  L(const P &a, const P &b) {
    push_back(a); push_back(b);
  }
};

typedef vector<P> G;

struct C {
  P p; double r;
  C(const P &p, double r) : p(p), r(r) { }
};
int ccw(P a, P b, P c) {
  b -= a; c -= a;
  if (cross(b, c) > 0)   return +1;       // counter clockwise
  if (cross(b, c) < 0)   return -1;       // clockwise
  if (dot(b, c) < 0)     return +2;       // c--a--b on line
  if (norm(b) < norm(c)) return -2;       // a--b--c on line
  return 0;
}
bool intersectLL(const L &l, const L &m) {
  return abs(cross(l[1]-l[0], m[1]-m[0])) > EPS || // non-parallel
         abs(cross(l[1]-l[0], m[0]-l[0])) < EPS;   // same line
}
bool intersectLS(const L &l, const L &s) {
  return cross(l[1]-l[0], s[0]-l[0])*       // s[0] is left of l
         cross(l[1]-l[0], s[1]-l[0]) < EPS; // s[1] is right of l
}
bool intersectLP(const L &l, const P &p) {
  return abs(cross(l[1]-p, l[0]-p)) < EPS;
}
bool intersectSS(const L &s, const L &t) {
  return ccw(s[0],s[1],t[0])*ccw(s[0],s[1],t[1]) <= 0 &&
         ccw(t[0],t[1],s[0])*ccw(t[0],t[1],s[1]) <= 0;
}
bool intersectSP(const L &s, const P &p) {
  return abs(s[0]-p)+abs(s[1]-p)-abs(s[1]-s[0]) < EPS; // triangle inequality
}

P projection(const L &l, const P &p) {
  double t = dot(p-l[0], l[0]-l[1]) / norm(l[0]-l[1]);
  return l[0] + t*(l[0]-l[1]);
}
double distanceLP(const L &l, const P &p) {
  return abs(p - projection(l, p));
}
double distanceLL(const L &l, const L &m) {
  return intersectLL(l, m) ? 0 : distanceLP(l, m[0]);
}
double distanceLS(const L &l, const L &s) {
  if (intersectLS(l, s)) return 0;
  return min(distanceLP(l, s[0]), distanceLP(l, s[1]));
}
double distanceSP(const L &s, const P &p) {
  const P r = projection(s, p);
  if (intersectSP(s, r)) return abs(r - p);
  return min(abs(s[0] - p), abs(s[1] - p));
}
double distanceSS(const L &s, const L &t) {
  if (intersectSS(s, t)) return 0;
  return min(min(distanceSP(s, t[0]), distanceSP(s, t[1])),
      min(distanceSP(t, s[0]), distanceSP(t, s[1])));
}
P crosspoint(const L &l, const L &m) {
  double A = cross(l[1] - l[0], m[1] - m[0]);
  double B = cross(l[1] - l[0], l[1] - m[0]);
  if (abs(A) < EPS && abs(B) < EPS) return m[0]; // same line
  if (abs(A) < EPS) assert(false); // !!!PRECONDITION NOT SATISFIED!!!
  return m[0] + B / A * (m[1] - m[0]);
}
typedef double Weight;
struct Edge{
  int src, dst;
  Weight weight;
  Edge(int src, int dst, Weight weight) :
    src(src), dst(dst), weight(weight) {}
};
bool operator < (const Edge &e, const Edge &f) {
  return e.weight != f.weight ? e.weight > f.weight : // !!INVERSE!!
    e.src != f.src ? e.src < f.src : e.dst < f.dst;
}
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;

typedef vector<Weight> Array;
typedef vector<Array> Matrix;

/* dijkstra: return distance, prev: O(ElogV) */
pair<vector<Weight>,vector<int> > dijkstra(const Graph &g, int s){
  int n = g.size();
  vector<Weight> dist(n, INF); dist[s] = 0;
  vector<int> prev(n, -1);
  priority_queue<Edge> que;
  for(que.push(Edge(-2,s,0)); !que.empty();){
    Edge e = que.top(); que.pop();
    if(prev[e.dst] != -1) continue;
    prev[e.dst] = e.src;
    FORIT(f,g[e.dst]){
      if(dist[f->dst] > e.weight + f->weight){
        dist[f->dst] = e.weight + f->weight;
        que.push(Edge(f->src, f->dst, dist[f->dst]));
      }
    }
  }
  return make_pair(dist, prev);
}

int main(){
  int na,nb;
  while(cin>>na>>nb){
    vector<P> pa(na);
    vector<P> pb(nb);
    REP(i,na){
      int x,y; cin>>x>>y;
      pa[i] = P(x,y);
    }
    REP(i,nb){
      int x,y; cin>>x>>y;
      pb[i] = P(x,y);
    }
    L la = L(pa[0],pa[1]);
    double da = abs(pa[0]-pa[1]);
    L lb = L(pb[0],pb[1]);
    double db = abs(pb[0]-pb[1]);
    Graph graph1(na);
    REP(i,na)FOR(j,i+1,na){
      L l = L(pa[i],pa[j]);
      if(!intersectSS(l,lb)){
        graph1[i].push_back(Edge(i,j,abs(pa[i]-pa[j])));
        graph1[j].push_back(Edge(j,i,abs(pa[i]-pa[j])));
      }
    }
    pair<vector<Weight>,vector<int> > dijk1 = dijkstra(graph1, 0);
    double len1 = dijk1.first[1];
    double ans1 = len1 + db;
    Graph graph2(nb);
    REP(i,nb)FOR(j,i+1,nb){
      L l = L(pb[i],pb[j]);
      if(!intersectSS(l,la)){
        graph2[i].push_back(Edge(i,j,abs(pb[i]-pb[j])));
        graph2[j].push_back(Edge(j,i,abs(pb[i]-pb[j])));
      }
    }
    pair<vector<Weight>,vector<int> > dijk2 = dijkstra(graph2, 0);
    double len2 = dijk2.first[1];
    double ans2 = len2 + da;
    double ans = min(ans1,ans2);
    if(ans >= INF-EPS){
      cout<<-1<<endl;
    }else{
      printf("%.12lf\n",ans);
    }
  }
  return 0;
}

Measured memory_bytes: 34000