#include <bits/stdc++.h>
using namespace std;
#define REP(i, n)           for(int(i)=0;(i)<(n);++(i))

int solve(int D, const string &s){
    int d = 99;
    vector<int> v;
    REP(i,14){
        if(s[i] == 'x'){
            if(d > 0) d++; else { v.push_back(d); d = +1; }
        } else {
            if(d < 0) d--; else { v.push_back(d); d = -1; }
        }
    }
    if(d < 0) v.push_back(d);
    v.push_back(99);
    int n = v.size();
    if(n == 1) return D;

    int res = 0;
    for(int i = 0; i < n; i += 2){
        if(v[i] > D){
            if(i > 0)   res = max(res, -v[i-1]+D);
            if(i < n-1) res = max(res, -v[i+1]+D);
        } else {
            res = max(res, v[i]-v[i-1]-v[i+1]);
        }
    }
    return res;
}

int main(){
    int D; cin >> D;
    string s,t;
    cin >> s >> t;
    s += t;
    cout << solve(D,s) << endl;
    return 0;
}