#include<stdio.h>

#include<iostream>

#include<string.h>

#include<queue>

using namespace std;

#define inf 99999999

#define maxn 50010

#define maxm maxn*51

int n,e,head[maxn],pre[maxm],nex[maxm];

double cost[maxm],dis[maxn];

bool vis[maxn];

queue<int> q;

void add(int u,int v,double c)

{

    pre[e]=v;

    nex[e]=head[u];

    cost[e]=c;

    head[u]=e++;

}

double spfa(int st,int end)

{

    memset(dis,,sizeof(dis));

    dis[st]=;

    q.push(st);

    while(!q.empty())

    {

        int u=q.front();

        vis[u]=;

        q.pop();

        for(int i=head[u];i!=-;i=nex[i])

            if(dis[pre[i]]<dis[u]*(-cost[i]))

            {

                dis[pre[i]]=dis[u]*(-cost[i]);

                if(!vis[pre[i]])

                {

                    q.push(pre[i]);

                    vis[pre[i]]=;

                }

            }

    }

    return dis[end];

}

int main()

{

    while(scanf("%d",&n)!=EOF)

    {

        e=;

        memset(head,-,sizeof(head));

        for(int i=;i<=n;i++)

        {

            int m;

            scanf("%d",&m);

            for(int j=;j<m;j++)

            {

                int u,c;

                scanf("%d%d",&u,&c);

                    add(i,u,c*1.0/);

            }

        }

        int st,end,tot;

        scanf("%d%d%d",&st,&end,&tot);

        double px=spfa(st,end);

        if(px==)

            printf("IMPOSSIBLE!\n");

        else

            printf("%.2f\n",(-px)*tot);  

    }

    return ;

}