#include<cassert>
#include <bits/stdc++.h>
#ifndef IO_HPP
#define IO_HPP
#include<iostream>
#include<vector>
#include<queue>
#include<stack>
#include<array>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
using namespace std;
template<typename T1,typename T2>istream &operator>>(istream&,pair<T1,T2>&);
template<typename...Args>istream &operator>>(istream&,tuple<Args...>&a);
template<typename T>istream &operator>>(istream&is,vector<T>&a);
template<typename T,size_t N>istream &operator>>(istream&is,array<T,N>&a);
template<typename T1,typename T2>
istream &operator>>(istream&is,pair<T1,T2>&a){
  is>>a.first>>a.second;
  return is;
}
template<size_t pos,typename...Args>
void read_tuple(istream&is,tuple<Args...>&a){
  if constexpr(pos<tuple_size<tuple<Args...>>::value){
    is>>get<pos>(a);
    read_tuple<pos+1>(is,a);
  }
}
template<typename...Args>
istream &operator>>(istream&is,tuple<Args...>&a){
  read_tuple<0>(is,a);
  return is;
}
template<typename T>
istream &operator>>(istream&is,vector<T>&a){
  for(T&x:a)is>>x;
  return is;
}
template<typename T,size_t N>
istream &operator>>(istream&is,array<T,N>&a){
  for(T&x:a)is>>x;
  return is;
}
template<typename T1,typename T2>ostream &operator<<(ostream&os,const pair<T1,T2>&);
template<typename...Args>ostream &operator<<(ostream&os,const tuple<Args...>&);
template<typename T>ostream &operator<<(ostream&os,const vector<T>&);
template<typename T,typename Seq,typename Comp>ostream &operator<<(ostream&os,priority_queue<T,Seq,Comp>);
template<typename T>ostream &operator<<(ostream&os,queue<T>);
template<typename T>ostream &operator<<(ostream&os,deque<T>);
template<typename T>ostream &operator<<(ostream&os,stack<T>);
template<typename T,size_t N>ostream &operator<<(ostream&os,const array<T,N>&);
template<typename Key,typename Val,typename Comp>ostream &operator<<(ostream&os,const map<Key,Val,Comp>&);
template<typename Key,typename Val,typename Hash>ostream &operator<<(ostream&os,const unordered_map<Key,Val,Hash>&);
template<typename T,typename Comp>ostream &operator<<(ostream&os,const set<T,Comp>&);
template<typename T,typename Comp>ostream &operator<<(ostream&os,const multiset<T,Comp>&);
template<typename T,typename Hash>ostream &operator<<(ostream&os,const unordered_set<T,Hash>&);
template<typename T1,typename T2>
ostream &operator<<(ostream&os,const pair<T1,T2>&a){
  os<<a.first<<' '<<a.second;
  return os;
}
template<size_t pos,typename...Args>
void write_tuple(ostream&os,const tuple<Args...>&a){
  if constexpr(pos<tuple_size<tuple<Args...>>::value){
    if constexpr(pos>0)os<<' ';
    os<<get<pos>(a);
    write_tuple<pos+1>(os,a);
  }
}
template<typename...Args>
ostream &operator<<(ostream&os,const tuple<Args...>&a){
  write_tuple<0>(os,a);
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,const vector<T>&a){
  os<<'{';
  for(int i=0;i<(int)a.size();i++){
    os<<a[i];
    if(i+1!=a.size())os<<',';
  }
  os<<'}';
  return os;
}
template<typename T,typename Seq,typename Comp>
ostream &operator<<(ostream&os,priority_queue<T,Seq,Comp>a){
  os<<'{';
  if(!a.empty()){
    os<<a.top();a.pop();
    while(!a.empty()){
      os<<',';
      os<<a.top();
      a.pop();
    }
  }
  os<<'}';
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,queue<T>a){
  os<<'{';
  if(!a.empty()){
    os<<a.front();a.pop();
    while(!a.empty()){
      os<<',';
      os<<a.front();
      a.pop();
    }
  }
  os<<'}';
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,deque<T>a){
  os<<'{';
  if(!a.empty()){
    os<<a.front();a.pop_front();
    while(!a.empty()){
      os<<',';
      os<<a.front();
      a.pop_front();
    }
  }
  os<<'}';
  return os;
}
template<typename T>
ostream &operator<<(ostream&os,stack<T>a){
  os<<'{';
  if(!a.empty()){
    os<<a.top();a.pop();
    while(!a.empty()){
      os<<',';
      os<<a.top();
      a.pop();
    }
  }
  os<<'}';
  return os;
}
template<typename T,size_t N>
ostream &operator<<(ostream&os,const array<T,N>&a){
  os<<'{';
  for(int i=0;i<(int)a.size();i++){
    os<<a[i];
    if(i+1!=a.size())os<<',';
  }
  os<<'}';
  return os;
}
template<typename Key,typename Val,typename Comp>
ostream &operator<<(ostream&os,const map<Key,Val,Comp>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<"{["<<itr->first<<","<<itr->second<<']';
  while(++itr!=a.end())os<<",["<<itr->first<<','<<itr->second<<']';
  os<<'}';
  return os;
}
template<typename Key,typename Val,typename Hash>
ostream &operator<<(ostream&os,const unordered_map<Key,Val,Hash>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<"{["<<itr->first<<","<<itr->second<<']';
  while(++itr!=a.end())os<<",["<<itr->first<<','<<itr->second<<']';
  os<<'}';
  return os;
}
template<typename T,typename Comp>
ostream &operator<<(ostream&os,const set<T,Comp>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<'{'<<*itr;
  while(++itr!=a.end())os<<','<<*itr;
  os<<'}';
  return os;
}
template<typename T,typename Comp>
ostream &operator<<(ostream&os,const multiset<T,Comp>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<'{'<<*itr;
  while(++itr!=a.end())os<<','<<*itr;
  os<<'}';
  return os;
}
template<typename T,typename Hash>
ostream &operator<<(ostream&os,const unordered_set<T,Hash>&a){
  if(a.empty()){
    os<<"{}";
    return os;
  }
  auto itr=a.begin();
  os<<'{'<<*itr;
  while(++itr!=a.end())os<<','<<*itr;
  os<<'}';
  return os;
}
#endif
using namespace std;
using ll=long long;
using ull=unsigned long long;
using P=pair<ll,ll>;
template<typename T>using minque=priority_queue<T,vector<T>,greater<T>>;
template<typename T>bool chmax(T &a,const T &b){return (a<b?(a=b,true):false);}
template<typename T>bool chmin(T &a,const T &b){return (a>b?(a=b,true):false);}
template<typename T1,typename T2>void operator++(pair<T1,T2>&a,int){a.first++,a.second++;}
template<typename T1,typename T2>void operator--(pair<T1,T2>&a,int){a.first--,a.second--;}
template<typename T>void operator++(vector<T>&a,int){for(auto &i:a)i++;}
template<typename T>void operator--(vector<T>&a,int){for(auto &i:a)i--;}
#define overload3(_1,_2,_3,name,...) name
#define rep1(i,n) for(int i=0;i<(int)(n);i++)
#define rep2(i,l,r) for(int i=(int)(l);i<(int)(r);i++)
#define rep(...) overload3(__VA_ARGS__,rep2,rep1)(__VA_ARGS__)
#define reps(i,l,r) rep2(i,l,r)
#define all(x) x.begin(),x.end()
#define pcnt(x) __builtin_popcountll(x)
#define fin(x) return cout<<(x)<<'\n',static_cast<void>(0)
#define yn(x) cout<<((x)?"Yes\n":"No\n")
#define uniq(x) sort(all(x)),x.erase(unique(all(x)),x.end())
template<typename T>
inline int fkey(vector<T>&z,T key){return lower_bound(z.begin(),z.end(),key)-z.begin();}
ll myceil(ll a,ll b){return (a+b-1)/b;}
template<typename T,size_t n,size_t id=0>
auto vec(const int (&d)[n],const T &init=T()){
  if constexpr (id<n)return vector(d[id],vec<T,n,id+1>(d,init));
  else return init;
}
#ifdef LOCAL
#include<debug.h>
#define SWITCH(a,b) (a)
#else
#define debug(...) static_cast<void>(0)
#define debugg(...) static_cast<void>(0)
#define SWITCH(a,b) (b)
#endif
struct Timer{
  clock_t start;
  Timer(){
    start=clock();
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout<<fixed<<setprecision(16);
  }
  inline double now(){return (double)(clock()-start)/1000;}
  #ifdef LOCAL
  ~Timer(){
    cerr<<"time:";
    cerr<<now();
    cerr<<"ms\n";
  }
  #endif
}timer;
void SOLVE();
int main(){
  int testcase=1;
  //cin>>testcase;
  for(int i=0;i<testcase;i++){
    SOLVE();
  }
}
template<typename T=int>
struct Edge{
  int from,to;
  T weight;
  int index;
  Edge(int from_,int to_,T weight_=T(),int index_=-1):from(from_),to(to_),weight(weight_),index(index_){}
  Edge():from(-1),to(-1),weight(),index(-1){}
  friend std::ostream &operator<<(std::ostream &os,const Edge&e){
    os<<'[';
    os<<"from:"<<e.from;
    os<<"to:"<<e.to;
    os<<"weight:"<<e.weight;
    os<<"index:"<<e.index;
    os<<']';
    return os;
  }
};
template<typename T=int>
struct Graph{
private:
  int n;
  std::vector<Edge<T>>edge;
  std::vector<Edge<T>>g;
  std::vector<int>ptr;
  bool directed;
  struct graph_range{
    using iterator=typename std::vector<Edge<T>>::iterator;
    iterator l,r;
    iterator begin()const{return l;}
    iterator end()const{return r;}
    int size()const{return r-l;}
    Edge<T> &operator[](int i)const{return l[i];}
  };
  struct const_graph_range{
    using iterator=typename std::vector<Edge<T>>::const_iterator;
    iterator l,r;
    iterator begin()const{return l;}
    iterator end()const{return r;}
    int size()const{return r-l;}
    const Edge<T> &operator[](int i)const{return l[i];}
  };
public:
  Graph(int n_,bool dir_):n(n_),directed(dir_){}
  Graph():n(0){}
  Graph(int n_,bool dir_,const std::vector<Edge<T>>&e):n(n_),directed(dir_),edge(e){build();}
  template<bool weighted=false,bool index=1>
  void read(int m){
    edge.reserve(m);
    for(int i=0;i<m;i++){
      int u,v;
      std::cin>>u>>v;
      T w;
      if constexpr(index)u--,v--;
      if constexpr(weighted)std::cin>>w;
      else w=1;
      edge.emplace_back(u,v,w,i);
    }
    build();
  }
  void add_edge(int u,int v){
    assert(0<=u&&u<n);
    assert(0<=v&&v<n);
    int id=edge.size();
    edge.emplace_back(u,v,1,id);
  }
  void add_edge(int u,int v,T w){
    assert(0<=u&&u<n);
    assert(0<=v&&v<n);
    int id=edge.size();
    edge.emplace_back(u,v,w,id);
  }
  void add_edge(int u,int v,T w,int index){
    assert(0<=u&&u<n);
    assert(0<=v&&v<n);
    edge.emplace_back(u,v,w,index);
  }
  void add_edge(Edge<T>e){
    edge.emplace_back(e);
  }
  void build(){
    std::vector<int>cnt(n+1,0);
    for(const Edge<T>&e:edge){
      cnt[e.from+1]++;
      if(!directed)cnt[e.to+1]++;
    }
    for(int i=1;i<=n;i++)cnt[i]+=cnt[i-1];
    ptr=cnt;
    g.resize(cnt[n]);
    for(const Edge<T>&e:edge){
      g[cnt[e.from]++]=e;
      if(!directed)g[cnt[e.to]++]=Edge<T>(e.to,e.from,e.weight,e.index);
    }
  }
  void reverse(){
    if(directed){
      for(Edge<T>&e:edge)std::swap(e.from,e.to);
      build();
    }
  }
  inline void to_directed(){
    directed=true;
    build();
  }
  inline void to_undirected(){
    directed=false;
    build();
  }
  void reserve(int m){edge.reserve(m);}
  graph_range operator[](int i){return graph_range{g.begin()+ptr[i],g.begin()+ptr[i+1]};}
  const_graph_range operator[](int i)const{return const_graph_range{g.begin()+ptr[i],g.begin()+ptr[i+1]};}
  const Edge<T>& get_edge(int i)const{return edge[i];}
  std::vector<Edge<T>>get_edges()const{return edge;}
  inline bool is_directed()const{return directed;}
  inline int size()const{return n;}
  inline int edge_size()const{return edge.size();}
  typename std::vector<Edge<T>>::iterator begin(){return edge.begin();}
  typename std::vector<Edge<T>>::iterator end(){return edge.end();}
  typename std::vector<Edge<T>>::const_iterator begin()const{return edge.begin();}
  typename std::vector<Edge<T>>::const_iterator end()const{return edge.end();}
};
#define NO {cout<<"-1\n";exit(0);}
ll solve(vector<ll>a,ll k){
  int n=a.size();
  if(n%2==1){
    vector<ll>coef(n);
    coef[0]=0;
    rep(i,1,n){
      coef[i]=a[i]-coef[i-1];
    }
    ll x=a[0]-coef[0]-coef[n-1];
    if(x<0||x%2==1)NO
    x/=2;
    if(1<=x&&x<k)NO
    ll res=0;
    debug(a,x,k);
    res+=(x+k*2-1)/(k*2);
    a[0]-=x;
    a[1]-=x;
    if(a[0]<0)NO
    if(a[1]<0)NO
    rep(i,1,n-1){
      if(a[i]>a[i+1])NO
      if(1<=a[i]&&a[i]<k)NO
      res+=(a[i]+k*2-1)/(k*2);
      a[i+1]-=a[i];
      a[i]=0;
    }
    assert(a[0]==a[n-1]);
    if(1<=a[0]&&a[0]<k)NO
    res+=(a[0]+k*2-1)/(k*2);
    return res;
  }
  {
    ll sum=0;
    rep(i,n){
      sum+=a[i]*(i&1?-1:1);
    }
    if(sum!=0)NO
  }
  ll mnx=0,mxx=a[0];
  {
    vector<pair<ll,ll>>coef(n);
    coef[0]={1,0};
    rep(i,1,n){
      coef[i].first=-coef[i-1].first;
      coef[i].second=a[i]-coef[i-1].second;
      if(coef[i].first==1){
        chmax(mnx,-coef[i].second);
      }
      else{
        chmin(mxx,coef[i].second);
      }
    }
  }
  if(mnx>mxx)NO
  ll res=1e18;
  auto yaru=[&](ll x)->void {
    if(x<=min(a[0],a[n-1])){
      ll now=(x+k*2-1)/(k*2);
      vector<ll>b(a);
      b[0]-=x;
      b.back()-=x;
      rep(i,n-1){
        if(b[i]>b[i+1]){
          return;
        }
        if(1<=b[i]&&b[i]<k){
          return;
        }
        now+=(b[i]+k*2-1)/(k*2);
        b[i+1]-=b[i];
        b[i]=0;
      }
      if(b[n-1]==0)chmin(res,now);
    }
  };
  auto yaru2=[&](ll x)->void {
    yaru(x);
    yaru(x%k);
    yaru(x%(k*2));
  };
  // yaru(0);
  // yaru(k);
  // yaru(k*2);
  // yaru2(a[0]);
  // yaru2(a[n-1]);
  yaru(0);
  rep(i,mnx,mxx+1)yaru(i);
  // mt19937_64 mt(0715);
  // debug(mnx,mxx);
  // while(timer.now()<1900){
  //   ll v=mt()%(mxx-mnx+1)+mnx;
  //   yaru(v);
  // }
  if(res==1e18)NO
  return res;
}
void SOLVE(){
  int n;
  ll k;
  cin>>n>>k;
  vector<ll>a(n);
  cin>>a;
  Graph g(n,false);
  g.read(n);
  vector<int>deg(n);
  rep(i,n)deg[i]=g[i].size();
  queue<int>que;
  ll ans=0;
  rep(i,n)if(deg[i]==1)que.push(i);
  while(!que.empty()){
    int x=que.front();que.pop();
    int u=-1;
    for(auto e:g[x])if(deg[e.to]>=2){
      assert(u==-1);
      u=e.to;
    }
    if(a[x]>a[u])fin(-1);
    if(1<=a[x]&&a[x]<k)fin(-1);
    ans+=(a[x]+k*2-1)/(k*2);
    a[u]-=a[x];
    if(--deg[u]==1)que.push(u);
  }
  vector<ll>b;
  debug(deg);
  int u=find_if(all(deg),[&](int x){return x>=2;})-deg.begin();
  assert(u!=n);
  int v=-1;
  for(auto e:g[u])if(deg[e.to]>=2){
    v=e.to;
    break;
  }
  assert(v!=-1);
  int pre=u;
  b.push_back(a[u]);
  while(u!=v){
    b.push_back(a[v]);
    for(auto e:g[v])if(deg[e.to]>=2&&e.to!=pre){
      pre=v;
      v=e.to;
      break;
    }
  }
  ans+=solve(b,k);
  cout<<ans<<endl;
}