#include <bits/stdc++.h>

#define il inline

#define _for(i, a, b) for (int i = (a); i <= (b); ++i)

#define _fore(i, u) for (int i = hd[u], v; v = tg[i], i; i = nx[i])

using namespace std;

struct {

    template <typename T>

    il operator T() {

        T x = 0, f = 1;

        char c = getchar();

        while (!isdigit(c)) (c == '-') && (f = -1), c = getchar();

        while (isdigit(c)) x = (x << 1) + (x << 3) + (c ^ 48), c = getchar();

        return f * x;

    }

} gi;



const int M = 1e6 + 5, N = 2e3 + 5, T = 4e3 + 5;

const int inf = 0x3f3f3f3f;



vector<tuple<int, int>> q[N];

vector<int> E[N];



int n, m;

int a[N];

int f[T];

int st[N], ed[N], tim, id[N];

int siz[N], hs[N];



int mxt;

int ans[M];



void pdfs(int u, int fa) {

    siz[u] = 1, st[u] = ++tim, id[tim] = u;

    for (auto v : E[u])

        if (v != fa) {

            pdfs(v, u), siz[u] += siz[v];

            if (siz[v] > siz[hs[u]]) hs[u] = v;

        }

    ed[u] = tim;

}



void upd(int v) {

    for (int i = mxt; i >= v; --i) f[i] = min(f[i], f[i - v] + 1);

}

void clear() { memset(f, 0x3f, sizeof(f)), f[0] = 0; }



void dfs(int u, int fa, int kep) {

    for (auto v : E[u])

        if (v != fa && v != hs[u]) dfs(v, u, 0);

    if (hs[u]) dfs(hs[u], u, 1);



    upd(a[u]);



    for (auto v : E[u])

        if (v != fa && v != hs[u])

            for (int p = st[v]; p <= ed[v]; ++p) upd(a[id[p]]);



    for (auto [t, id] : q[u]) ans[id] = (f[t] == inf ? -1 : f[t]);



    if (!kep) clear();

}



int main() {

    n = gi, m = gi;



    for (int i = 1; i <= n; ++i) a[i] = gi;

    for (int i = 1, u, v; i < n; ++i) u = gi, v = gi, E[u].push_back(v), E[v].push_back(u);



    for (int i = 1, u, t; i <= m; ++i) u = gi, t = gi, q[u].emplace_back(t, i), mxt = max(mxt, t);



    pdfs(1, 0);

    clear();

    dfs(1, 0, 1);



    for (int i = 1; i <= m; ++i) printf("%d\n", ans[i]);



    return 0;

}