#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <vector>



using namespace std;

int main(){
	int d;char s1[1000],s2[1000];scanf("%d%s%s",&d,s1,s2);
	strcat(s1,s2);int ans=0,len=strlen(s1);
	vector<int> v,vcnt;
	int cnt=0;
	v.push_back(-1);vcnt.push_back(0);
	for(int i=0;i<len;i++){
		if(s1[i]=='o'&&v[v.size()-1]==0)vcnt[v.size()-1]++;
		else if(s1[i]=='o'){v.push_back(0);vcnt.push_back(1);} 
		if(s1[i]=='x'&&v[v.size()-1]==1)vcnt[v.size()-1]++;
		else if(s1[i]=='x'){v.push_back(1);vcnt.push_back(1);} 
		if(v.size()>2&&v[v.size()-1]==0&&(i==len-1||s1[i+1]=='x')&&vcnt[v.size()-2]<=d){
			cnt=vcnt[v.size()-1]+min(vcnt[v.size()-2],d)+vcnt[v.size()-3];
			if(ans<cnt)ans=cnt;
			
			
		}
	}  
	for(int i=0;i<v.size();i++){
		if(v[i]==0){
			if(ans<vcnt[i]+d)ans=vcnt[i]+d;
		}
	}
	
	if(ans<d)ans=d;
	 
	printf("%d\n",ans);
	
	
	
	return 0;
}