前缀和秒了
k = int(input())
s, a, b = input().split()
n = len(s)
g = [0] * n
cnt = 0
for i in range(n):
if s[i] == b: cnt += 1
g[i] = cnt # g[i]为前i个数中b的个数
res = 0
for i in range(n - k + 1):
if s[i] == a:
j = i + k - 1
res += g[n - 1] - g[j - 1] # 区间[j, n - 1]中b的个数
print(res)