#define _USE_MATH_DEFINES

#include <iostream>
#include <iomanip>

#include <algorithm>
#include <cmath>

#include <string>
#include <list>
#include <queue>
#include <vector>
#include <complex>

/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) std::cout<<(p)<<std::endl;
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////

int main(void){
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed;//
    //cout << setprecision(6);//
	
	int D;
	cin>>D;
	string A,B;
	cin>>A>>B;
	int count = 0;
	int ter = -1;
	
	int oDay[8];
	int xDay[8];
	rep(i,8){
		oDay[i]=0;
		xDay[i]=0;
	}
	string S = A+B;
	for(unsigned int i=0;i<S.size();++i){
		if(S[i]=='o'){//休日
			++oDay[count];
			ter = 0;
		}
		else if(S[i] =='x'){//平日
			if(ter == 0){
				++count;
				++xDay[count-1];
				ter = 1;
			}
			else if(ter == 1){
				++xDay[count-1];
			}
		}
	}
	if(S[13]=='x'){
		--count;
	}
	if(count==-1){
		P(D);
		return 0;
	}
	int ans = 0;
	int temp;
	for(int i=0;i<=count;++i){
		if(i==0){
			temp = D+oDay[i];
		}else{
			if(xDay[i-1] <= D){
				temp = oDay[i-1]+xDay[i-1]+oDay[i];
			}else{
				temp = D + oDay[i];
			}
		}
		if(ans < temp) ans = temp;
		if(i==count){
			temp = oDay[i] + D;
		}else{
			if(xDay[i] <= D){
				temp = oDay[i] + xDay[i] + oDay[i+1];
			}else{
				temp = oDay[i] + D;
			}
		}
		if(ans < temp) ans = temp;
	}
	P(ans);
	return 0;
}