#include "bits/stdc++.h"
#define REP(i,n,N) for(int i=(n); i<(N); i++)
#define RREP(i,n,N) for(ll i=(N-1); i>=n; i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define ALL(v) (v).begin(),(v).end()
#define p(s) cout<<(s)<<endl
#define p2(a,b) cout<<(a)<<" "<<(b)<<endl
typedef long long ll;
using namespace std;
const ll inf=1e18;

int D;
string s,s1;
int sum[20];
int main(){
    cin>>D;
    string heijitu = "xxxxxxxxxxxxxx";
    cin>>s>>s1;
    s=heijitu+s+s1+heijitu;
    int n=s.size();
    int ans=0;
    REP(i,0,n-D){
        if(s.substr(i,D).find("o")!=string::npos) continue;
        int cnt=0;
        RREP(j,0,i) {
            if(s[j]=='o'){
                cnt++;
            }else{
                break;
            }
        }
        REP(j,i+D,n){
            if(s[j]=='o'){
                cnt++;
            }else{
                break;
            }
        }
        ans= max(ans,cnt+D);
    }
    p(ans);
    return 0;
}