#include <stdio.h>#include <vector>#include <algorithm>#include <string.h>#include <map>#include <iostream>using namespace std;struct point{int id,size;point(){}point(int _p,int _size){id=_p;size=_size;}bool operator<(const point &a)const{return size>a.size;}};const int maxn=1100;const double INF=1e20;int value;int v[maxn*2],next[maxn*2],start[maxn],nume;void bui(int a,int b){nume++;v[nume]=b;next[nume]=start[a];start[a]=nume;}double dp[maxn][maxn],need[maxn];int size[maxn];void dfs(int now,int pa){vector<point> a;for(int p=start[now];p!=-1;p=next[p])if(v[p]!=pa){dfs(v[p],now);a.push_back(point(v[p],size[v[p]]));}size[now]=1;for(int i=0;i<=size[now];i++)dp[now][i]=INF;dp[now][1]=0;for(int p=0;p<a.size();p++){for(int i=size[now]+1;i<=size[now]+a[p].size;i++)dp[now][i]=INF;size[now]+=a[p].size;for(int i=size[now]-a[p].size;i>0;i--)for(int j=1;j<=a[p].size;j++)dp[now][i+j]=min(dp[now][i+j],dp[now][i]+dp[a[p].id][j]);}for(int i=1;i<=size[now];i++){if(i==1&&size[now]!=1)dp[now][i]=INF;else dp[now][i]+=need[now]/(value+i-1);}}map<string,int> id;int main(){int n;while(scanf("%d%d",&n,&value)!=EOF){id.clear();for(int i=1;i<=n;i++)start[i]=-1;nume=0;for(int i=1;i<=n;i++){static string a;cin>>a;id[a]=i;scanf("%lf",&need[i]);}for(int i=1;i<n;i++){static string a,b;cin>>a>>b;bui(id[b],id[a]);}string c;cin>>c;dfs(id[c],-1);double ans=INF;for(int i=1;i<=size[id[c]];i++)ans=min(ans,dp[id[c]][i]);printf("%.2fn",ans);}}
