import sys

def dfs(u):
    used[u] = True
    c = w = cw = ww = cww = 0
    for v in e[u]:
        if not used[v]:
            res = dfs(v)
            cww += res[4] + c * res[3] + w * res[2] + cw * res[1] + ww * res[0]
            if s[u] == 'w':
                cww += c * res[1] + w * res[0]
            c += res[0]
            w += res[1]
            cw += res[2]
            ww += res[3]
    if s[u] == 'c':
        c += 1
        cww += ww
    else:
        cww += cw
        cw += c
        ww += w
        w += 1
    return [c,w,cw,ww,cww]

sys.setrecursionlimit(150000)
n = int(input())
s = input()
e = [[] for i in range(n)]
used = [False] * n
for i in range(n-1):
    a, b = map(int,input().split(' '))
    a -= 1
    b -= 1
    e[a].append(b)
    e[b].append(a)
print(dfs(0)[4])