#include<bits/stdc++.h>

 #define INF 0x7fffffff

 #define LL long long

 #define N 100005

 using namespace std;

 inline int ra()

 {

     int x=,f=; char ch=getchar();

     while (ch<'' || ch>'') {if (ch=='-') f=-; ch=getchar();}

     while (ch>='' && ch<='') {x=x*+ch-''; ch=getchar();}

     return x*f;

 }

 double p,ans,a[],f[][],g[][];

 int main()

 {

     int n=ra(),m=ra();

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

         scanf("%lf",&a[i]);

     int st=ra();

     scanf("%lf",&p);

     memset(f,0xc2,sizeof(f));

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

         f[i][i]=;

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

     {

         int x=ra(),y=ra();

         f[x][y]=a[y];

     }

     double tmp=p;

     for (int T=; T<=; T++,tmp*=tmp)

     {

         memset(g,0xc2,sizeof(g));

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

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

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

                     g[i][j]=max(g[i][j],f[i][k]+f[k][j]*tmp);

         memcpy(f,g,sizeof(f));

     }

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

         ans=max(ans,f[st][i]);

     printf("%.1lf\n",ans*p+a[st]);

     return ;

 }