D - Alter Altar https://atcoder.jp/contests/abc174/tasks/abc174_d

abc174_d.py

N = int(input())
S = input()

prefixW = [0] * (N + 1)         # iまでの白の個数
totalW = 0                      # 全体で白が何個あるか

for i in range(N):
    prefixW[i + 1] = prefixW[i]
    if S[i] == 'W':
        prefixW[i + 1] += 1
        totalW += 1

ans = N
for i in range(N + 1):
    leftW = prefixW[i]
    rightR = (N - i) - (totalW - leftW)
    ans = min(ans, max(leftW, rightR))

print(ans)