# include <cstdio>

# include <cstring>

# include <cstdlib>

# include <iostream>

# include <vector>

# include <queue>

# include <stack>

# include <map>

# include <set>

# include <cmath>

# include <algorithm>

using namespace std;

# define lowbit(x) ((x)&(-x))

# define pi acos(-1.0)

# define eps 1e-

# define MOD

# define INF

# define mem(a,b) memset(a,b,sizeof(a))

# define FOR(i,a,n) for(int i=a; i<=n; ++i)

# define FO(i,a,n) for(int i=a; i<n; ++i)

# define bug puts("H");

# define lch p<<,l,mid

# define rch p<<|,mid+,r

# define mp make_pair

# define pb push_back

typedef pair<int,int> PII;

typedef vector<int> VI;

# pragma comment(linker, "/STACK:1024000000,1024000000")

typedef long long LL;

int Scan() {

    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;

}

void Out(int a) {

    if(a<) {putchar('-'); a=-a;}

    if(a>=) Out(a/);

    putchar(a%+'');

}

const int N=;

//Code begin...

double dp[N][N][N], val[N], Pow[N], sum[N];

int E[N*][];

int main ()

{

    int n, m, s, u, v;

    double p, ans=;

    scanf("%d%d",&n,&m);

    FOR(i,,n) scanf("%lf",val+i);

    scanf("%d%lf",&s,&p);

    FOR(i,,m) scanf("%d%d",&E[i][],&E[i][]);

    FOR(i,,n) FOR(j,,n) FOR(k,,n) dp[i][j][k]=-1e18;

    Pow[]=; FOR(i,,n+) Pow[i]=Pow[i-]*p;

    FOR(i,,n) sum[i]=dp[][i][i]=val[i];

    FOR(i,,n) FOR(j,,n) FOR(k,,m) dp[i][j][E[k][]]=max(dp[i][j][E[k][]],dp[i-][j][E[k][]]+val[E[k][]]*Pow[i]);

    FOR(j,,n) FOR(i,,n) sum[j]=max(sum[j],(dp[i][j][j]-val[j]*Pow[i])/(-Pow[i]));

    FOR(i,,n) FOR(j,,n) if (dp[i][s][j]>=) ans=max(ans,max(dp[i][s][j],dp[i][s][j]+sum[j]*Pow[i]-val[j]*Pow[i]));

    printf("%.1f\n",ans);

    return ;

}