子串简写

代码

k=int(input())
s,c1,c2=input().split()
pre=[0]*len(s)
ans=0
for i in range(len(s)):
    pre[i]=pre[i-1]
    if c1==s[i]:
        pre[i]+=1
    elif c2==s[i] and i+1-k>=0:
        ans+=pre[i-k+1]
print(ans)