#pragma GCC target("avx") #pragma GCC optimize("O3") #pragma GCC optimize("unroll-loops") #include using namespace std; using ll=long long; using datas=pair; using ddatas=pair; using tdata=pair; using vec=vector; using mat=vector; using pvec=vector; using pmat=vector; #define For(i,a,b) for(i=a;i<(ll)b;++i) #define bFor(i,b,a) for(i=b,--i;i>=(ll)a;--i) #define rep(i,N) For(i,0,N) #define rep1(i,N) For(i,1,N) #define brep(i,N) bFor(i,N,0) #define brep1(i,N) bFor(i,N,1) #define all(v) (v).begin(),(v).end() #define allr(v) (v).rbegin(),(v).rend() #define vsort(v) sort(all(v)) #define vrsort(v) sort(allr(v)) #define uniq(v) vsort(v);(v).erase(unique(all(v)),(v).end()) #define endl "\n" #define popcount __builtin_popcountll #define eb emplace_back #define print(x) cout< ostream& operator<<(ostream& os,const pair& p){return os<<"("< ostream& operator<<(ostream& os,const vector& v){ os<<"{";bool f=false; for(auto& x:v){if(f)os<<",";os< ostream& operator<<(ostream& os,const set& v){ os<<"{";bool f=false; for(auto& x:v){if(f)os<<",";os< ostream& operator<<(ostream& os,const multiset& v){ os<<"{";bool f=false; for(auto& x:v){if(f)os<<",";os< ostream& operator<<(ostream& os,const map& v){ os<<"{";bool f=false; for(auto& x:v){if(f)os<<",";os< inline bool chmax(T& a,const T b){bool x=a inline bool chmin(T& a,const T b){bool x=a>b;if(x)a=b;return x;} #ifdef DEBUG void debugg(){cout<void debugg(const T& x,const Args&... args){cout<<" "< dijkstra(pmat& g,ll first){ priority_queue> que; que.emplace(0,first); vector dist(g.size(),inf); while(!que.empty()){ datas x=que.top();que.pop(); if(!chmin(dist[x.second],x.first))continue; for(auto next:g[x.second]){ next.first+=x.first; if(!chmin(dist[next.second],next.first+1))continue; que.emplace(next); } } return dist; } ll N,M,K,H,W,A,B,C,D; int main(){ startupcpp(); ll i,j; cin>>N>>A>>B>>K; --A;--B; vec x(N); rep(i,N){ cin>>x[i]; x[i]*=2; } cin>>M; pmat g(N),rg(N); rep(i,M){ ll a,b,c; cin>>a>>b>>c; c*=2; --a;--b; g[a].emplace_back(c+(x[a]+x[b])/2,b); rg[b].emplace_back(c+(x[a]+x[b])/2,a); } mat dp(K,vec(N,inf)),bf(K,vec(N,inf)); dp[0][A]=x[A]/2; rep(i,K-1){ rep(j,N){ for(auto [d,to]:g[j]){ if(chmin(dp[i+1][to],dp[i][j]+d))bf[i+1][to]=j; } } } auto dist=dijkstra(rg,B); vec anslist(N); rep(i,N)anslist[i]=dist[i]+dp[K-1][i]; i=min_element(all(anslist))-anslist.begin(); if(anslist[i]>=inf){ print("Impossible"); return 0; } assert(dp[K-1][i]