#ifndef LOCAL
#define FAST_IO
#endif

// ============
#include <bits/stdc++.h>
#define OVERRIDE(a, b, c, d, ...) d
#define REP2(i, n) for (i32 i = 0; i < (i32)(n); ++i)
#define REP3(i, m, n) for (i32 i = (i32)(m); i < (i32)(n); ++i)
#define REP(...) OVERRIDE(__VA_ARGS__, REP3, REP2)(__VA_ARGS__)
#define PER2(i, n) for (i32 i = (i32)(n)-1; i >= 0; --i)
#define PER3(i, m, n) for (i32 i = (i32)(n)-1; i >= (i32)(m); --i)
#define PER(...) OVERRIDE(__VA_ARGS__, PER3, PER2)(__VA_ARGS__)
#define ALL(x) begin(x), end(x)
#define LEN(x) (i32)(x.size())
using namespace std;
using u32 = unsigned int;
using u64 = unsigned long long;
using i32 = signed int;
using i64 = signed long long;
using f64 = double;
using f80 = long double;
using pi = pair<i32, i32>;
using pl = pair<i64, i64>;
template <typename T>
using V = vector<T>;
template <typename T>
using VV = V<V<T>>;
template <typename T>
using VVV = V<V<V<T>>>;
template <typename T>
using VVVV = V<V<V<V<T>>>>;
template <typename T>
using PQR = priority_queue<T, V<T>, greater<T>>;
template <typename T>
bool chmin(T &x, const T &y) {
    if (x > y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T>
bool chmax(T &x, const T &y) {
    if (x < y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T>
i32 lob(const V<T> &arr, const T &v) {
    return (i32)(lower_bound(ALL(arr), v) - arr.begin());
}
template <typename T>
i32 upb(const V<T> &arr, const T &v) {
    return (i32)(upper_bound(ALL(arr), v) - arr.begin());
}
template <typename T>
V<i32> argsort(const V<T> &arr) {
    V<i32> ret(arr.size());
    iota(ALL(ret), 0);
    sort(ALL(ret), [&](i32 i, i32 j) -> bool {
        if (arr[i] == arr[j]) {
            return i < j;
        } else {
            return arr[i] < arr[j];
        }
    });
    return ret;
}
#ifdef INT128
using u128 = __uint128_t;
using i128 = __int128_t;
#endif
[[maybe_unused]] constexpr i32 INF = 1000000100;
[[maybe_unused]] constexpr i64 INF64 = 3000000000000000100;
struct SetUpIO {
    SetUpIO() {
#ifdef FAST_IO
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
#endif
        cout << fixed << setprecision(15);
    }
} set_up_io;
void scan(char &x) { cin >> x; }
void scan(u32 &x) { cin >> x; }
void scan(u64 &x) { cin >> x; }
void scan(i32 &x) { cin >> x; }
void scan(i64 &x) { cin >> x; }
void scan(f64 &x) { cin >> x; }
void scan(string &x) { cin >> x; }
template <typename T>
void scan(V<T> &x) {
    for (T &ele : x) {
        scan(ele);
    }
}
void read() {}
template <typename Head, typename... Tail>
void read(Head &head, Tail &...tail) {
    scan(head);
    read(tail...);
}
#define CHAR(...)     \
    char __VA_ARGS__; \
    read(__VA_ARGS__);
#define U32(...)     \
    u32 __VA_ARGS__; \
    read(__VA_ARGS__);
#define U64(...)     \
    u64 __VA_ARGS__; \
    read(__VA_ARGS__);
#define I32(...)     \
    i32 __VA_ARGS__; \
    read(__VA_ARGS__);
#define I64(...)     \
    i64 __VA_ARGS__; \
    read(__VA_ARGS__);
#define F64(...)     \
    f64 __VA_ARGS__; \
    read(__VA_ARGS__);
#define STR(...)        \
    string __VA_ARGS__; \
    read(__VA_ARGS__);
#define VEC(type, name, size) \
    V<type> name(size);       \
    read(name);
#define VVEC(type, name, size1, size2)    \
    VV<type> name(size1, V<type>(size2)); \
    read(name);
// ============

#ifdef DEBUGF
#else
#define DBG(...) (void)0
#endif

// ============
#include <iostream>
#include <cassert>
#include <vector>
template <typename T>
struct Edge {
    using W = T;
    int from, to, id;
    W weight;
    Edge<T> rev() const {
        return Edge<T>{to, from, id, weight};
    }
};
template <typename T>
void debug(const Edge<T> &e) {
    std::cerr << e.from << " -> " << e.to << " id = " << e.id << std::cerr << " weight = ";
    debug(e.weight);
}
template <typename T = int, bool DIR = false>
class Graph {
public:
    using E = Edge<T>;
    using W = T;
    static constexpr bool DIRECTED = DIR;
    struct Adjacency {
        using Iter = typename std::vector<E>::iterator;
        Iter be, en;
        Iter begin() const { return be; }
        Iter end() const { return en; }
        int size() const { return (int)std::distance(be, en); }
        E &operator[](int idx) const { return be[idx]; }
    };
    struct ConstAdjacency {
        using Iter = typename std::vector<E>::const_iterator;
        Iter be, en;
        Iter begin() const { return be; }
        Iter end() const { return en; }
        int size() const { return (int)std::distance(be, en); }
        const E &operator[](int idx) const { return be[idx]; }
    };

private:
    int n, m;
    std::vector<E> edges, csr;
    std::vector<int> sep;
    bool built;

public:
    Graph(int n) : n(n), m(0), built(false) {}
    int v() const { return n; }
    int e() const { return m; }
    int add_vertex() {
        return n++;
    }
    void add_edge(int from, int to, W weight = 1) {
        assert(0 <= from && from < n && 0 <= to && to < n);
        edges.emplace_back(E{from, to, m++, weight});
    }
    void build() {
        sep.assign(n + 1, 0);
        csr.resize(DIRECTED ? m : 2 * m);
        for (const E &e : edges) {
            ++sep[e.from + 1];
            if (!DIRECTED) {
                ++sep[e.to + 1];
            }
        }
        for (int i = 0; i < n; ++i) {
            sep[i + 1] += sep[i];
        }
        std::vector<int> c = sep;
        for (const E &e : edges) {
            csr[c[e.from]++] = e;
            if (!DIRECTED) {
                csr[c[e.to]++] = e.rev();
            }
        }
        built = true;
    }
    Adjacency operator[](int v) {
        assert(built && 0 <= v && v < n);
        return Adjacency{csr.begin() + sep[v], csr.begin() + sep[v + 1]};
    }
    ConstAdjacency operator[](int v) const {
        assert(built && 0 <= v && v < n);
        return ConstAdjacency{csr.begin() + sep[v], csr.begin() + sep[v + 1]};
    }
};
// ============
// ============

#include <utility>

constexpr bool is_prime(unsigned n) {
    if (n == 0 || n == 1) {
        return false;
    }
    for (unsigned i = 2; i * i <= n; ++i) {
        if (n % i == 0) {
            return false;
        }
    }
    return true;
}

constexpr unsigned mod_pow(unsigned x, unsigned y, unsigned mod) {
    unsigned ret = 1, self = x;
    while (y != 0) {
        if (y & 1) {
            ret = (unsigned)((unsigned long long)ret * self % mod);
        }
        self = (unsigned)((unsigned long long)self * self % mod);
        y /= 2;
    }
    return ret;
}

template <unsigned mod>
constexpr unsigned primitive_root() {
    static_assert(is_prime(mod), "`mod` must be a prime number.");
    if (mod == 2) {
        return 1;
    }

    unsigned primes[32] = {};
    int it = 0;
    {
        unsigned m = mod - 1;
        for (unsigned i = 2; i * i <= m; ++i) {
            if (m % i == 0) {
                primes[it++] = i;
                while (m % i == 0) {
                    m /= i;
                }
            }
        }
        if (m != 1) {
            primes[it++] = m;
        }
    }
    for (unsigned i = 2; i < mod; ++i) {
        bool ok = true;
        for (int j = 0; j < it; ++j) {
            if (mod_pow(i, (mod - 1) / primes[j], mod) == 1) {
                ok = false;
                break;
            }
        }
        if (ok) return i;
    }
    return 0;
}

// y >= 1
template <typename T>
constexpr T safe_mod(T x, T y) {
    x %= y;
    if (x < 0) {
        x += y;
    }
    return x;
}

// y != 0
template <typename T>
constexpr T floor_div(T x, T y) {
    if (y < 0) {
        x *= -1;
        y *= -1;
    }
    if (x >= 0) {
        return x / y;
    } else {
        return -((-x + y - 1) / y);
    }
}

// y != 0
template <typename T>
constexpr T ceil_div(T x, T y) {
    if (y < 0) {
        x *= -1;
        y *= -1;
    }
    if (x >= 0) {
        return (x + y - 1) / y;
    } else {
        return -(-x / y);
    }
}

// b >= 1
// returns (g, x) s.t. g = gcd(a, b), a * x = g (mod b), 0 <= x < b / g
// from ACL
template <typename T>
std::pair<T, T> extgcd(T a, T b) {
    a = safe_mod(a, b);
    T s = b, t = a, m0 = 0, m1 = 1;
    while (t) {
        T u = s / t;
        s -= t * u;
        m0 -= m1 * u;
        std::swap(s, t);
        std::swap(m0, m1);
    }
    if (m0 < 0) {
        m0 += b / s;
    }
    return std::pair<T, T>(s, m0);
}

// b >= 1
// returns (g, x, y) s.t. g = gcd(a, b), a * x + b * y = g, 0 <= x < b / g, |y| < max(2, |a| / g)
template <typename T>
std::tuple<T, T, T> extgcd2(T a, T b) {
    T _a = safe_mod(a, b);
    T quot = (a - _a) / b;
    T x00 = 0, x01 = 1, y0 = b;
    T x10 = 1, x11 = -quot, y1 = _a;
    while (y1) {
        T u = y0 / y1;
        x00 -= u * x10;
        x01 -= u * x11;
        y0 -= u * y1;
        std::swap(x00, x10);
        std::swap(x01, x11);
        std::swap(y0, y1);
    }
    if (x00 < 0) {
        x00 += b / y0;
        x01 -= a / y0;
    }
    return std::tuple<T, T, T>(y0, x00, x01);
}

// gcd(x, m) == 1
template <typename T>
T inv_mod(T x, T m) {
    return extgcd(x, m).second;
}
// ============

i64 loop(i64 k, const V<i64> &a) {
    DBG(k, a);
    i32 n = LEN(a);
    if (n % 2 == 1) {
        V<i64> ops(n, 0);
        REP(i, 1, n) {
            ops[i] = a[i] - ops[i - 1];
        }
        i64 more = a[0] - (ops[0] + ops[n - 1]);
        if (more % 2 != 0) {
            return -1;
        }
        more /= 2;
        REP(i, n) {
            if (i % 2 == 0) {
                ops[i] += more;
            } else {
                ops[i] -= more;
            }
        }
        DBG(ops);
        i64 ans = 0;
        REP(i, n) {
            if (ops[i] < 0 || (0 < ops[i] && ops[i] < k)) {
                return -1;
            }
            ans += ceil_div(ops[i], 2 * k);
        }
        return ans;
    }
    assert(n % 2 == 0);
    V<i64> ops(n, 0);
    REP(i, 1, n) {
        ops[i] = a[i] - ops[i - 1];
    }
    if (ops[0] + ops[n - 1] != a[0]) {
        return -1;
    }
    DBG(k, n, a, ops);
    // +x -x +x -x ... +x -x
    i64 low = -INF64, high = INF64;
    REP(i, n / 2) {
        chmax(low, -ops[2 * i]);
        chmin(high, ops[2 * i + 1]);
    }
    if (low > high) {
        return -1;
    }
    // mendou
    i64 ans = 0;
    REP(i, n) {
        ans += ceil_div(ops[i], 2 * k);
    }
    // tasukete!!!!
    V<tuple<i64, i64, i32>> add;
    REP(i, n / 2) {
        {
            i64 val = ops[2 * i];
            i64 md = safe_mod(val, 2 * k);
            if (md == 0) {
                add.emplace_back(1, 2 * k, 1);
            } else if (md >= 2) {
                add.emplace_back(2 * k + 1 - md, 2 * k, 1);
            }
        }
        {
            i64 val = ops[2 * i + 1];
            i64 md = safe_mod(val, 2 * k);
            if (md != 0) {
                add.emplace_back(md, 2 * k, -1);
            }
        }
    }
    DBG(ans, low, high, add);
    V<tuple<i64, i64, i32>> ban;
    if (k > 1) {
        REP(i, n / 2) {
            {
                i64 val = ops[2 * i];
                // 1 <= val + x < k
                // 1 - val <= x < k - val
                i64 l = 1 - val, r = k - val;
                if (floor_div(l, 2 * k) < floor_div(r - 1, 2 * k)) {
                    ban.emplace_back(safe_mod(l, 2 * k), 2 * k, floor_div(l, 2 * k));
                    ban.emplace_back(0, safe_mod(r, 2 * k), floor_div(r, 2 * k));
                } else {
                    ban.emplace_back(safe_mod(l, 2 * k), safe_mod(r - 1, 2 * k) + 1, floor_div(l, 2 * k));
                }
            }
            {
                i64 val = ops[2 * i];
                // 1 <= val - x < k
                // val - k < x <= val - 1
                i64 l = val - k + 1, r = val; // [l, r)
                if (floor_div(l, 2 * k) < floor_div(r - 1, 2 * k)) {
                    ban.emplace_back(safe_mod(l, 2 * k), 2 * k, floor_div(l, 2 * k));
                    ban.emplace_back(0, safe_mod(r, 2 * k), floor_div(r, 2 * k));
                } else {
                    ban.emplace_back(safe_mod(l, 2 * k), safe_mod(r - 1, 2 * k) + 1, floor_div(l, 2 * k));
                }
            }
        }
    }
    DBG(ban);
    V<i64> pos;
    pos.push_back(0);
    pos.push_back(2 * k);
    for (auto [l, r, v] : add) {
        pos.push_back(l);
        pos.push_back(r);
    }
    for (auto [l, r, v] : ban) {
        pos.push_back(l);
        pos.push_back(r);
    }
    pos.push_back(safe_mod(low, 2 * k));
    pos.push_back(safe_mod(high + 1, 2 * k));
    sort(ALL(pos));
    pos.erase(unique(ALL(pos)), end(pos));
    i32 m = LEN(pos);
    VV<i32> push(m), remov(m);
    V<i32> lu(m, 0);
    for (auto [l, r, v] : add) {
        lu[lob(pos, l)] += v;
        lu[lob(pos, r)] -= v;
    }
    REP(i, m - 1) {
        lu[i + 1] += lu[i];
    }
    for (auto [l, r, v] : ban) {
        l = lob(pos, l);
        r = lob(pos, r);
        push[l].push_back(v);
        remov[r].push_back(v);
    }
    multiset<i64> ms;
    i32 diff = INF;
    REP(i, m - 1) {
        for (i64 v : remov[i]) {
            ms.erase(ms.find(v));
        }
        for (i64 v : push[i]) {
            ms.insert(v);
        }
        i64 md = pos[i];
        // md + 2 * k * x >= low
        // x >= ceil((low - md) / (2 * k))
        i64 x = ceil_div(low - md, 2 * k);
        bool ok = false;
        while (md + 2 * x * k <= high) {
            if (!ms.contains(x)) {
                ok = true;
                break;
            }
            ++x;
        }
        if (ok) {
            chmin(diff, lu[i]);
        }
    }
    ans += diff;
    return ans;
}

void solve() {
    I32(n);
    I64(k);
    VEC(i64, a, n);
    Graph<> g(n);
    REP(i, n) {
        I32(u, v);
        --u;
        --v;
        g.add_edge(u, v);
    }
    g.build();
    V<i32> deg(n);
    REP(i, n) {
        deg[i] = LEN(g[i]);
    }
    queue<i32> que;
    i64 ans = 0;
    REP(i, n) {
        if (deg[i] == 1) {
            que.push(i);
        }
    }
    while (!que.empty()) {
        i32 v = que.front();
        que.pop();
        i32 u = -1;
        for (auto e : g[v]) {
            if (deg[e.to] >= 2) {
                u = e.to;
            }
        }
        assert(u != -1);
        if (a[v] < 0 || (0 < a[v] && a[v] < k)) {
            cout << -1 << '\n';
            return;
        }
        a[u] -= a[v];
        ans += ceil_div(a[v], 2 * k);
        if (--deg[u] == 1) {
            que.push(u);
        }
    }
    DBG(deg, a, ans);
    REP(v, n) {
        if (deg[v] != 2) {
            continue;
        }
        V<i64> arr;
        i32 cur = v;
        i32 prv = -1;
        while (true) {
            arr.push_back(a[cur]);
            deg[cur] = -1;
            i32 to = -1;
            for (auto e : g[cur]) {
                if (deg[e.to] == 2 && e.to != prv) {
                    to = e.to;
                    break;
                }
            }
            if (to == -1) {
                break;
            }
            prv = exchange(cur, to);
        }
        i64 sub = loop(k, arr);
        if (sub == -1) {
            cout << -1 << '\n';
            return;
        }
        ans += sub;
    }
    cout << ans << '\n';
}

int main() {
    i32 t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }
}