# haru: pypy
import sys
input = sys.stdin.readline
II = lambda : int(input())
MI = lambda : map(int, input().split())
LI = lambda : [int(a) for a in input().split()]
SI = lambda : input().rstrip()
LLI = lambda n : [[int(a) for a in input().split()] for _ in range(n)]
LSI = lambda n : [input().rstrip() for _ in range(n)]
MI_1 = lambda : map(lambda x:int(x)-1, input().split())
LI_1 = lambda : [int(a)-1 for a in input().split()]

mod = 998244353
inf = 1001001001001001001
ordalp = lambda s : ord(s)-65 if s.isupper() else ord(s)-97
ordallalp = lambda s : ord(s)-39 if s.isupper() else ord(s)-97
yes = lambda : print("Yes")
no = lambda : print("No")
yn = lambda flag : print("Yes" if flag else "No")

prinf = lambda ans : print(ans if ans < 1000001001001001001 else -1)
alplow = "abcdefghijklmnopqrstuvwxyz"
alpup = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
alpall = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
URDL = {'U':(-1,0), 'R':(0,1), 'D':(1,0), 'L':(0,-1)}
DIR_4 = [[-1,0],[0,1],[1,0],[0,-1]]
DIR_8 = [[-1,0],[-1,1],[0,1],[1,1],[1,0],[1,-1],[0,-1],[-1,-1]]
DIR_BISHOP = [[-1,1],[1,1],[1,-1],[-1,-1]]
prime60 = [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59]
sys.set_int_max_str_digits(0)
# sys.setrecursionlimit(10**6)
# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')

from collections import defaultdict,deque
from heapq import heappop,heappush
from bisect import bisect_left,bisect_right
DD = defaultdict
BSL = bisect_left
BSR = bisect_right

"""
namori 
k <= x <= 2k を減らせる
うーん、
"""


n, k = MI()
a = LI()

d = [0] * n 
e = [[] for i in range(n)]

for i in range(n):
    u, v = MI_1()
    e[u].append(v)
    e[v].append(u)
    
    d[u] += 1
    d[v] += 1


que = [i for i in range(n) if d[i] == 1]

ans = 0

for u in que:
    for v in e[u]:
        if d[v] > 0:
            p = v
            break
    
    a[p] -= a[u]
    if a[p] < 0:
        print(-1)
        exit()
    
    d[u] = 0
    d[p] -= 1
    if d[p] == 1:
        que.append(p)
    
    if 0 < a[u] < k:
        print(-1)
        exit()
    else:
        # たぶんあってる
        ans += ((a[u] - 1) // (2 * k)) + 1

# que に入っていない頂点が cycle

for i in range(n):
    if d[i] > 0:
        u = i
        break

# print(ans, a)

s = u
cyc = []
p = -1
while True:
    # print(u)
    cyc.append(u)
    for v in e[u]:
        if d[v] > 0 and v != p:
            nxt = v
            break
    if nxt == s:
        break
    p, u = u, nxt


b = [a[u] for u in cyc]
# print(ans, b)


m = len(b)

"""
b[i] = x[i-1] + x[i] とする

x[i] は
d[i] + t : even
d[i] - t : odd
"""

d = [0] * m
for i in range(1, m):
    d[i] = b[i] - d[i - 1]


sgn = [1, -1]
if m % 2 == 1:
    # b[0] = 2 t + d[m - 1]
    
    t = b[0] - d[-1]
    if t % 2 == 1:
        print(-1)
        exit()
    t //= 2
    
    
    d = [d[i] + (t * sgn[i & 1]) for i in range(m)]
    # これ回数
    
    for i in range(m):
        if 0 < d[i] < k:
            print(-1)
            exit()
        else:
            ans += ((d[i] - 1) // (2 * k)) + 1
    print(ans)
else:
    if d[-1] != b[0]:
        print(-1)
        exit()
    
    # 最小化がひつようめう
    
    assert False