WA代码；过样例了

#include<bits/stdc++.h>
using namespace std;
int fl(int k,const string&s,char a1,char a2){
	int n=s.length(),count=0;
	vector<int>a1_;
	for(int i=0;i<n;i++)if(s[i]==a1)a1_.push_back(i);
	for(int i=k-1;i<n;i++)
		if(s[i]==a2){
			int left=i-k****tart=0,end=a1_.size()-1;
			while(start<=end)
			{int mid=start+(end-start)/2;
			if(a1_[mid]<left) start=mid+1;else end=mid-1;}
			count+=a1_.size()-start;
		}
	return count;
}
int main(){
	int k;string s;char a1,a2;cin>>k>>s>>a1>>a2;
	int l=fl(k,s,a1,a2);
	cout<<l<<endl;
	return 0;
}