n = int(input())
s = input()
g = [[] for _ in range(n)]
for _ in range(n-1):
    a,b = map(int,input().split())
    g[a-1].append(b-1)
    g[b-1].append(a-1)

order = [0]
parent = [-1]*n
for v in order:
    for c in g[v]:
        if parent[v] == c: continue
        order.append(c)
        parent[c] = v

ans = 0
C = [0]*n
W = [0]*n
cs = s.count("c")
ws = n-cs
for v in order[::-1]:
    if s[v]=="c":
        C[v] += 1
    else:
        ans += C[v]*(ws-W[v]-1) + W[v]*(cs-C[v])
        W[v] += 1
    p = parent[v]
    if p==-1: break
    if s[p] == "w":
        ans += C[p]*W[v] + W[p]*C[v]
    C[p] += C[v]
    W[p] += W[v]
print(ans)