Yet Another Broken Keyboard[双指针]

#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
const int N=2e5+10;


int main()
{
    int n,k;cin>>n>>k;
    set<char>st;string s,s1;
    cin>>s;
    char tmp;
    for(int i=0;i<k;++i) cin>>tmp,st.insert(tmp);
    ll ans=0;
    for(int i=0;i<s.size();++i){
        int j=i;
        while(j<s.size()&&st.count(s[j])) ++j;
        ans+=1ll*(j-i)*(j-i+1)/2;
        i=j;
    }
    cout<<ans<<endl;
    return 0;
}