#include <vector>
#include <list>
#include <map>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <queue>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream>
#include <cstdio>
#include <climits>
#include <complex>
#include <cstdint>
#include <tuple>

#define M_PI       3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util

//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
struct Edge {
    int to, cost;
    Edge(int to, int cost): to(to), cost(cost) {}
};

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vector<Edge>> AdjList;
AdjList graph;

const int INF = 100000000;

LL N, M;
int h[155];
int main() {
    cout << fixed << setprecision(15);

    int D; cin >> D;
    string C1, C2; cin >> C1 >> C2;

    for(int i=0; i<C1.size(); ++i){
        if(C1[i] == 'o') h[i+20]=1;
        else h[i+20] = 0;
    }

    for(int i=0; i<C2.size(); ++i){
        if(C2[i] == 'o') h[i+27]=1;
        else h[i+27] = 0;
    }
    //for(int i=0; i<14; ++i) cout << h[i] << endl;
    int ans = 0;
    for(int i=0; i<155; ++i){

        int hh[100]={0};
        for(int j=0; j<100; ++j) hh[j] = h[j];

        for(int j=i; j<100; ++j){
            if(hh[j] == 0){
                for(int k=j; k<min(100, j+D); ++k){
                    if(hh[k]==1) break;
                    hh[k] = 1;
                }
                break;
            }
        }

        int sum = 0;
        for(int j=i; j<100; ++j){
            if(hh[j] == 0) break;
            sum++;
        }
        ans = max(ans, sum);
    }
    cout << ans << endl;
    return 0;
}