#include<bits/stdc++.h>
#define LL long long
#define DB double
int MOD;
#define ls(x) x << 1
#define rs(x) x << 1 | 1
#define lowbit(x) x & (-x)
#define PII pair<int, int>
#define MP make_pair
#define VI vector<int>
#define VII vector<int>::iterator
#define all(x) x.begin(), x.end()
#define EB emplace_back
#define SI set<int>
#define SII set<int>::iterator
#define QI queue<int>
#define fi first
#define se second
using namespace std;
template<typename T> void chkmn(T &a, const T &b) { (a > b) && (a = b); }
template<typename T> void chkmx(T &a, const T &b) { (a < b) && (a = b); }
int inc(const int &a, const int &b) { return a + b >= MOD ? a + b - MOD : a + b; }
int dec(const int &a, const int &b) { return a - b < 0 ? a - b + MOD : a - b; }
int mul(const int &a, const int &b) { return 1LL * a * b % MOD; }
int sqr(const int &a) { return 1LL * a * a % MOD; }
void Inc(int &a, const int &b) { ((a += b) >= MOD) && (a -= MOD); }
void Dec(int &a, const int &b) { ((a -= b) < 0) && (a += MOD); }
void Mul(int &a, const int &b) { a = 1LL * a * b % MOD; }
void Sqr(int &a) { a = 1LL * a * a % MOD; }
int qwqmi(int x, int k = MOD - 2)
{
    int res = 1;
    while(k)
    {
        if(k & 1) Mul(res, x);
        Sqr(x), k >>= 1;
    }
    return res;
}
template<typename T> void read(T &x)
{
	x = 0;
	int f = 1;
	char ch = getchar();
	while(!isdigit(ch))
	{
		if(ch == '-')
			f = -1;
		ch = getchar();
	}
	while(isdigit(ch))
	{
		x = (x << 1) + (x << 3) + (ch ^ 48);
		ch = getchar();
	}
	x= x * f;
}
const int N = 1e5 + 5;
const int INF = 1e9 + 7;
int n, m, K, invK, w[N];
int qwq[N], qwqinv[N];
int a[N], b[N];
vector<int> G[N];
struct Shiroha
{
	int root;
	int nowsz;
	int maxsz;
	int sz[N];
	int vis[N];
	unordered_map<int, int> mp1, mp2; 
	void get_root(int u, int fa)
	{
		sz[u] = 1;
		int maxx = 0;
		for(auto v : G[u])
		{
			if(v == fa || vis[v])
				continue;
			get_root(v, u);
			sz[u] += sz[v];
			chkmx(maxx, sz[v]);
		}
		chkmx(maxx, nowsz - sz[u]);
		if(maxx < maxsz)
		{
			maxsz = maxx;
			root = u;
		}
	}
	void modify(int u, int fa, int w1, int w2, int dep, int op)
	{
		mp1[w1] += op;
		mp2[mul(dec(m, w2), qwqinv[dep + 1])] += op;
		for(auto v : G[u])
		{
			if(v == fa || vis[v])
				continue;
			modify(v, u, inc(w1, mul(w[v], qwq[dep + 1])), inc(mul(w2, K), w[v]), dep + 1, op);
		}
	}
	void dfs(int u, int fa, int w1, int w2, int dep)
	{
		a[u] += mp2[w1];
		b[u] += mp1[mul(dec(m, w2), qwqinv[dep + 1])]; 
		for(auto v : G[u])
		{
			if(v == fa || vis[v])
				continue;
			dfs(v, u, inc(w1, mul(w[v], qwq[dep + 1])), inc(mul(w2, K), w[v]), dep + 1);
		}
	}
	void calc(int u)
	{
//		mp1.clear();
//		mp2.clear();
		while(!mp1.empty()) mp1.erase(mp1.begin());
		while(!mp2.empty()) mp2.erase(mp2.begin());
		modify(u, 0, w[u], w[u], 0, 1);
		a[u] += mp2[0];
		b[u] += mp1[m];
		for(auto v : G[u])
		{
			if(vis[v])
				continue;
			modify(v, u, inc(w[u], mul(w[v], K)), inc(mul(w[u], K), w[v]), 1, -1);
			dfs(v, u, w[v], w[v], 0);
			modify(v, u, inc(w[u], mul(w[v], K)), inc(mul(w[u], K), w[v]), 1, 1);
		}
	}
	void PDC(int u, int dep)
	{
//		cerr << dep << '\n';
		vis[u] = 1, calc(u);
		for(auto v : G[u])
		{
			if(vis[v])
				continue;
			get_root(v, u);
			nowsz = sz[v];
			maxsz = INF;
			get_root(v, u);
			PDC(root, dep + 1);
		}
	}
}umi;
LL ans;
int main()
{
	read(n), read(MOD), read(K), read(m);
	for(int i = 1; i <= n; ++i) read(w[i]);
	for(int i = 1; i < n; ++i)
	{
		int x, y;
		read(x), read(y);
		G[x].EB(y), G[y].EB(x);
	}
	qwq[0] = qwqinv[0] = 1;
	invK = qwqmi(K);
	for(int i = 1; i < N; ++i)
	{
		qwq[i] = mul(qwq[i - 1], K);
		qwqinv[i] = mul(qwqinv[i - 1], invK);
	}
	umi.PDC(1, 0);
	ans = 1LL * n * n * n;
	for(int i = 1; i <= n; ++i)
	{
		ans -= 3LL * n * a[i];
		ans += 1LL * a[i] * b[i];
		ans += 1LL * a[i] * a[i];
		ans += 1LL * b[i] * b[i]; 
	}
	cout << ans << '\n';
    return 0;
}