from itertools import *
from bisect import *
from collections import *
from heapq import *
import sys

sys.setrecursionlimit(10 ** 6)

def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def SI(): return sys.stdin.readline()[:-1]
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
int1 = lambda x: int(x) - 1
def MI1(): return map(int1, sys.stdin.readline().split())
def LI1(): return list(map(int1, sys.stdin.readline().split()))
p2D = lambda x: print(*x, sep="\n")
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def main():
    def dfs(u=0,pu=-1):
        for v in to[u]:
            if v==pu:continue
            dfs(v,u)
            w[u]+=w[v]+(s[v]=="w")*1
            ww[u]+=ww[v]+(s[v]=="w")*w[v]

    def dfs2(u=0,pu=-1):
        for v in to[u]:
            if v==pu:continue
            wv=w[v]+(s[v]=="w")*1
            ww[v]+=ww[u]-(ww[v]+(s[v]=="w")*w[v])+(w[u]-wv)*(s[u]=="w")
            w[v]+=w[u]-wv+(s[u]=="w")
            dfs2(v,u)


    n=II()
    s=SI()
    to=[[] for _ in range(n)]
    for _ in range(n-1):
        a,b=MI1()
        to[a].append(b)
        to[b].append(a)
    # 全方位DP
    # w[u]...頂点uのサブツリーのwの合計
    # ww[u]...頂点uのサブツリーのwwの合計
    w=[0]*n
    ww=[0]*n
    # 頂点0を根として子のサブツリーについて処理
    dfs()
    #print(w)
    #print(ww)
    # 親方向のサブツリーを加算していく
    dfs2()
    #print(w)
    #print(ww)
    # 文字がcの頂点におけるwwの数がcwwの数
    ans=0
    for u in range(n):
        if s[u]=="w":continue
        ans+=ww[u]
    print(ans)

main()