import sys
input = sys.stdin.readline

N = int(input())
S = list(input().rstrip())
allc, allw = 0, 0 
for i in range(N):
    S[i] = int(S[i] == "w")

allw = sum(S)
allc = N - allw

G = [[] for i in range(N)]
for i in range(N - 1):
    a, b = map(int, input().split())
    a, b = a - 1, b - 1
    G[a].append(b)
    G[b].append(a)


def non_rec_dfs(s):
    stack = []
    stack.append(s)
    par = [-1] * N
    sz = [[0] * 2 for i in range(N)]
    for i in range(N):
        sz[i][S[i]] += 1
    ans = 0
    while stack:
        u = stack.pop()
        if u >= 0:
            stack.append(~u)
            for v in G[u]:
                if v == par[u]:
                    continue
                par[v] = u
                stack.append(v)
        else:
            u = ~u
            if S[u]:
                ans += allc * (allw - 1)
                noww, nowc = allw, allc
                for v in G[u]:
                    if v == par[u]:
                        continue
                    ans -= sz[v][0] * sz[v][1]
                    nowc -= sz[v][0]
                    noww -= sz[v][1]
                ans -= nowc * (noww - 1)
            if par[u] != -1:
                sz[par[u]][0] += sz[u][0]
                sz[par[u]][1] += sz[u][1]

    return ans

print(non_rec_dfs(0))